The cerebral cortex is a rich and diverse structure that is the basis of intelligent behavior. One of the deepest mysteries of the function of cortex is that neural processing times are only about one hundred times as fast as the fastest response times for complex behavior. At the very least, this would seem to indicate that the cortex does massive amounts of parallel computation.This paper explores the hypothesis that an important part of the cortex can be modeled as (...) a connectionist computer that is especially suited for parallel problem solving. The connectionist computer uses a special representation, termed value unit encoding, that represents small subsets of parameters in a way that allows parallel access to many different parameter values. This computer can be thought of as computing hierarchies of sensorimotor invariants. The neural substrate can be interpreted as a commitment to data structures and algorithms that compute invariants fast enough to explain the behavioral response times. A detailed consideration of this model has several implications for the underlying anatomy and physiology. (shrink)

Cortical color blindness, or cerebral achromatopsia, has been likened by some authors to ''blindsight'' for color or an instance of ''covert'' processing of color. Recently, it has been shown that, although such patients are unable to identify or discriminate hue differences, they nevertheless show a striking ability to process wavelength differences, which can result in preserved sensitivity to chromatic contrast and motion in equiluminant displays. Moreover, visually evoked cortical potentials can still be elicited in response to chromatic (...) stimuli. We suggest that these demonstrations reveal intact residual processes rather than the operation of covert processes, where proficient performance is accompanied by a denial of phenomenal awareness. We sought evidence for such covert processes by conducting appropriate tests on achromatopsic subject M.S. An ''indirect'' test entailing measurement of reaction times for letter identification failed to reveal covert color processes. In contrast, in a forced choice oddity task for color, M.S. was unable to verbally indicate the position of the different color, but was surprisingly adept at making an appropriate eye movement to its location. This ''direct'' test thus revealed the possible covert use of chromatic differences. (shrink)

In his comments, Daniel Nicholls succeeds in saying more than a few things that I had scarcely realized about the ways in which I write and, therefore, of what I tend to take for granted. He sees in what I write a capacity ‘to utilize the “obvious” whilst at the same time saying something about it.’ Not every philosopher would take that as a compliment. Many philosophers and philosophies have quite other pretensions – to transcend the illusions of common (...) thought and perception towards a higher plane of metaphysical philosophy – or of physics and mathematics. I find that Nicholls’s remark both compliments and complements my work, however. The use of everyday language and example to disturb that everyday world to ‘bring it into high relief’ as Nicholls puts it – is a significant part of philosophical method. To juxtapose the esoteric language of metaphysics (being-in-itself, being-for-itself, being-for-others) with the everyday (what there is, living for oneself, living for others) is to haunt the everyday with a metaphysical imagination even as it reveals what metaphysics had erased from the coinage of common words. (I am thinking, of course, of Derrida’s remarks about Anatole France’s satirical view of philosophical language as coinage from which all marking has been erased). (shrink)

Cortical stimulation has been used for brain mapping for over a century, and a standard assumption is that stimulation interferes with task execution due to local effects at the stimulation site. Stimulation can however produce afterdischarges which interfere with functional localization and can lead to unwanted seizures. We previously showed that cognitive effort can terminate these afterdischarges, when termination thus occurs, there are electrocorticography changes throughout the cortex, not just at sites with afterdischarges or sites thought functionally important for (...) the cognitive task used, and thresholds for afterdischarges and functional responses can change among stimulation trials. We here show that afterdischarge termination can occur prior to overt performance of the cognitive tasks used to terminate them. These findings, taken together, demonstrate that task-related brain changes are not limited to one or a group of functional regions or a specific network, and not limited to the time directly surrounding overt task execution. Discrete locations, networks and times importantly underpin clinical behaviors. However, brain activity that is diffuse in location and extended in time also affect task execution and can affect brain mapping. This may in part reflect fluctuating levels of attention, engagement, or motivation during testing. (shrink)

In cortical neurones, analogue dendritic potentials are thought to be encoded into patterns of digital spikes. According to this view, neuronal codes and computations are based on the temporal patterns of spikes: spike times, bursts or spike rates. Recently, we proposed an ‘action potential waveform code’ for cortical pyramidal neurones in which the spike shape carries information. Broader somatic action potentials are reliably produced in response to higher conductance input, allowing for four times more information transfer than (...) spike times alone. This information is preserved during synaptic integration in a single neurone, as back‐propagating action potentials of diverse shapes differentially shunt incoming postsynaptic potentials and so participate in the next round of spike generation. An open question has been whether the information in action potential waveforms can also survive axonal conduction and directly influence synaptic transmission to neighbouring neurones. Several new findings have now brought new light to this subject, showing cortical information processing that transcends the classical models. BioEssays 29: 178–187, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

Since the discovery that the characteristics of dreaming sleep are far stronger in Stage 1 rapid eye movement sleep than in any other biological state, investigators have attempted to determine the relative responsibility of the tonic versus the phasic properties of REM sleep for the different characteristics of dreaming–features such as the amount of information in the dream report, the brightness and clarity of the visual images, shifts in thematic continuity, and incongruities of image and meaning. The present experiment is (...) designed to identify dream characteristics that are specifically associated with tonic changes in level of cortical activation within sleep. It samples reports of imagery and thought during spontaneous variations within one phase of the 24-h diurnal rhythm and across the REM-NREM sleep cycle in order to identify the independent and joint contributions of the two cycles to imagery and thought. The rising phase of the diurnal cycle in the late night and morning was estimated from clock time during the late night and early morning and was varied by delaying the sleep onset and waking time of the subjects. Considered together with other studies, the results suggest that the major determinant of vivid visual imagery and enhanced cognitive activity during sleep is a pattern of subcortical and cortical activation that is common to both the REM phase of the REM-NREM cycle and the activated phase of the 24-h diurnal wake-sleep cycle. (shrink)

Physical fatigue negatively affects postural control, resulting in impaired balance performance in young and older adults. Similar effects on postural control can be observed for mental fatigue mainly in older adults. Controversial results exist for young adults. There is a void in the literature on the effects of fatigue on balance and cortical activity. Therefore, this study aimed to examine the acute effects of PF and MF on postural sway and cortical activity. Fifteen healthy young adults aged 28 (...) ± 3 years participated in this study. MF and PF protocols comprising of an all-out repeated sit-to-stand task and a computer-based attention network test, respectively, were applied in random order. Pre and post fatigue, cortical activity and postural sway were tested during a challenging bipedal balance board task. Absolute spectral power was calculated for theta, alpha-2, beta-1, and beta-2 in frontal, central, and parietal regions of interest and baseline-normalized. Inference statistics revealed a significant time-by-fatigue interaction for CoPd and CoPv, and a significant main effect of time for CoP variability. Post hoc analyses showed a significant increase in CoPd and CoPv following PF but not MF. For cortical activity, a significant time-by-fatigue interaction was found for relative alpha-2 power in parietal areas. Post hoc tests indicated larger alpha-2 power increases after PF compared to MF. In addition, changes in parietal alpha-2 power and measures of postural sway did not correlate significantly, irrespective of the applied fatigue protocol. No significant changes were found for the other frequency bands, irrespective of the fatigue protocol and ROI under investigation. Thus, the applied PF protocol resulted in increased postural sway and CoP variability accompanied by enhanced alpha-2 power in the parietal ROI while MF led to increased CoP variability and alpha-2 power in our sample of young adults. Potential underlying cortical mechanisms responsible for the greater increase in parietal alpha-2 power after PF were discussed but could not be clearly identified as cause. Therefore, further future research is needed to decipher alternative interpretations. (shrink)

BackgroundResponsive neurostimulation has been utilized as a treatment for intractable epilepsy. The RNS System delivers stimulation in response to detected abnormal activity, via leads covering the seizure foci, in response to detections of predefined epileptiform activity with the goal of decreasing seizure frequency and severity. While thalamic leads are often implanted in combination with cortical strip leads, implantation and stimulation with bilateral thalamic leads alone is less common, and the ability to detect electrographic seizures using RNS System (...) thalamic leads is uncertain.ObjectiveThe present study retrospectively evaluated fourteen patients with RNS System depth leads implanted in the thalamus, with or without concomitant implantation of cortical strip leads, to determine the ability to detect electrographic seizures in the thalamus. Detailed patient presentations and lead trajectories were reviewed alongside electroencephalographic analyses.ResultsAnterior nucleus thalamic leads, whether bilateral or unilateral and combined with a cortical strip lead, successfully detected and terminated epileptiform activity, as demonstrated by Cases 2 and 3. Similarly, bilateral centromedian thalamic leads or a combination of one centromedian thalamic alongside a cortical strip lead also demonstrated the ability to detect electrographic seizures as seen in Cases 6 and 9. Bilateral pulvinar leads likewise produced reliable seizure detection in Patient 14. Detections of electrographic seizures in thalamic nuclei did not appear to be affected by whether the patient was pediatric or adult at the time of RNS System implantation. Sole thalamic leads paralleled the combination of thalamic and cortical strip leads in terms of preventing the propagation of electrographic seizures.ConclusionThalamic nuclei present a promising target for detection and stimulation via the RNS System for seizures with multifocal or generalized onsets. These areas provide a modifiable, reversible therapeutic option for patients who are not candidates for surgical resection or ablation. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Response to the CommentariesSean A. SpenceIn “Free Will in the Light of Neuropsychiatry,” I have attempted to present an argument from the perspective of materialist neuroscience, pushing the latter to its logical conclusion: that if the human nervous system is consistent in its properties, then the only place for “free will” is in the non-conscious processes which underpin conscious awareness. This argument I have based on two supports: (...) the “spatial” and the “temporal” (Spence, 1995).I have contended that the sense of freedom may be lost from certain actions, thereby implying that “freedom” is a sensory process distinct from action. This might be conceptualized as having a distinct functional distribution within the nervous system (a spatial distinction). I have also argued that conscious awareness of events occurs at a finite delay, the period of “neuronal adequacy,” after those events themselves. The point here is that neurochemical events giving rise to phenomenology are likely to require a period of time (no matter how short) for their genesis. The process is “on line” before we perceive it (a temporal distinction). Thus the “freedom to act,” a motor process, is necessarily non-conscious at the point of initiation.I am grateful to Frith, Libet and Stephens for their comments. In the most part our disagreements are likely to be those of emphasis, perhaps compounded by my uses of the term “free will.”I have no major disagreement with Frith’s comments. I would, however, emphasize that it is likely that the Bereitschaftspotential, or readiness potential, does tell us something regarding intended actions. It is, for example, absent prior to the involuntary movements seen in Tourette’s syndrome (Obeso et al. 1982), and reduced in akinetic patients with Parkinson’s disease (Dick et al. 1989) or medial frontal lobe lesions (Deecke et al. 1987). The latter groups have difficulty initiating intended, internally generated, actions.With regard to the suspicion of dualism in my diagrams, the point is well taken, but I am attempting to describe a functional distinction, the precise coordinates of which remain obscure. It may well be that “consciousness” occupies no fixed location but is an activity temporarily co-extant with that association cortex (or cortices) subserving the sensory modality (or modalities) experienced phenomenologically (see Bottini et al. 1995).I have based much of my “temporal” argument on Libet’s work, and I am grateful for his response. Nevertheless, I am still of the opinion that there is a central inconsistency with his argument that conscious control exerts a veto on acts which is somehow free of material constraints. [Indeed he states that he has “no quarrel with the view that even a fully voluntary act is initiated unconsciously (non-consciously)”.] Whereas I can [End Page 99] not prove that this power of veto does not evade neurochemical processes, or that it somehow acts instantaneously and independently, with no origin in unconscious processing, I can say that I have restricted myself to arguing within a materialist perspective, with the aim of “pushing it” to its extreme. The free will which Libet retains in consciousness is one which is unique, and unconstrained by materialism. It is dualism in disguise!Stephens implies that there is a non-argument regarding consciousness acting as an initiator of events. The suggestion is that no one would argue this. Yet this is Libet’s argument. It cannot be disproved, but it is clearly (internally) inconsistent with Libet’s own impressive body of work.As for the role of consciousness in action, I have, I think, invoked a position similar to that adopted by Stephens; that consciousness is more a sensory state, allowing awareness of actions, and their consequences, and perhaps thereby facilitating the function of non-conscious initiators. It is clearly necessary for the performance of the “higher” acts which we equate with human nature, but it is not sufficient. (Indeed many of the most creative insights have “emerged” into consciousness [see Boden, 1992].) The man with a paralyzed arm may wish for his arm to move but to no avail. Over time function may slowly reappear. But if his lesion is such that he is unaware of his impairment (anosognosia), then his prognosis may... (shrink)

Transcranial magnetic stimulation was used to test the functional role of parietal and prefrontal cortical regions activated during a playing card Guilty Knowledge Task. Single-pulse TMS was applied to 15 healthy volunteers at each of three target sites: left and right dorsolateral prefrontal cortex and midline parietal cortex. TMS pulses were applied at each of five latencies after the onset of a card stimulus. TMS applied to the parietal cortex exerted a latency-specific increase in inverse efficiency score and in (...) reaction time when subjects were instructed to lie relative to when asked to respond with the truth, and this effect was specific to when TMS was applied at 240 ms after stimulus onset. No effects of TMS were detected at left or right DLPFC sites. This manipulation with TMS of performance in a deception task appears to support a critical role for the parietal cortex in intentional false responding, particularly in stimulus selection processes needed to execute a deceptive response in the context of a GKT. However, this interpretation is only preliminary, as further experiments are needed to compare performance within and outside of a deceptive context to clarify the effects of deceptive intent. (shrink)

The cell assembly model of language posits that words are laid down in the cortex by discrete sets of neurons distributed over specific parts of the brain. The strong internal links of these “word webs” may not only bind articulatory and acoustic knowledge of a lexical item, they may also link word and meaning; for example, by connecting neuron populations related to word forms to those of actions and perceptions to which the words refer. Therefore, the cortical activation elicited (...) by words should reflect aspects of word meaning, a postulate that has received strong support from recent work using neurophysiological and metabolic imaging. Segalowitz & Lane make the point that this neurobiological model can also be used to predict reaction times in behavioral experiments, using the behavioral distinction between content and function words as an example. We acclaim their view, but warn that response times might be related to different mechanisms at the neuronal level, including the cortical distribution and internal connectivity of cell assemblies along with their mutual connections in the grammatical (syntactic and semantic) network. (shrink)

Knee proprioception deficits and neuroplasticity have been indicated following injury to the anterior cruciate ligament. Evidence is, however, scarce regarding brain response to knee proprioception tasks and the impact of ACL injury. This study aimed to identify brain regions associated with the proprioceptive sense of joint position at the knee and whether the related brain response of individuals with ACL reconstruction differed from that of asymptomatic controls. Twenty-one persons with unilateral ACL reconstruction of either the right or left (...) knee, as well as 19 controls matched for sex, age, height, weight and current activity level, performed a knee joint position sense test during simultaneous functional magnetic resonance imaging. Integrated motion capture provided real-time knee kinematics to activate test instructions, as well as accurate knee angles for JPS outcomes. Recruited brain regions during knee angle reproduction included somatosensory cortices, prefrontal cortex and insula. Neither brain response nor JPS errors differed between groups, but across groups significant correlations revealed that greater errors were associated with greater ipsilateral response in the anterior cingulate, supramarginal gyrus and insula. This is the first study to capture brain response using fMRI in relation to quantifiable knee JPS. Activated brain regions have previously been associated with sensorimotor processes, body schema and interoception. Our innovative paradigm can help to guide future research investigating brain response to lower limb proprioception. (shrink)

Transcranial Photobiomodulation has demonstrated its ability to alter electrophysiological activity in the human brain. However, it is unclear how tPBM modulates brain electroencephalogram networks and is related to human cognition. In this study, we recorded 64-channel EEG from 44 healthy humans before, during, and after 8-min, right-forehead, 1,064-nm tPBM or sham stimulation with an irradiance of 257 mW/cm2. In data processing, a novel methodology by combining group singular value decomposition with the exact low-resolution brain electromagnetic tomography was implemented and performed (...) on the 64-channel noise-free EEG time series. The gSVD+eLORETA algorithm produced 11 gSVD-derived principal components projected in the 2D sensor and 3D source domain/space. These 11 PCs took more than 70% weight of the entire EEG signals and were justified as 11 EEG brain networks. Finally, baseline-normalized power changes of each EEG brain network in each EEG frequency band were quantified during the first 4-min, second 4-min, and post tPBM/sham periods, followed by comparisons of frequency-specific power changes between tPBM and sham conditions. Our results showed that tPBM-induced increases in alpha powers occurred at default mode network, executive control network, frontal parietal network and lateral visual network. Moreover, the ability to decompose EEG signals into individual, independent brain networks facilitated to better visualize significant decreases in gamma power by tPBM. Many similarities were found between the cortical locations of SVD-revealed EEG networks and fMRI-identified resting-state networks. This consistency may shed light on mechanistic associations between tPBM-modulated brain networks and improved cognition outcomes. (shrink)

This study explored calibrating a large item bank for use in multidimensional health measurement with computerized adaptive testing, using both item responses and response time (RT) information. The Activity Measure for Post-Acute Care is a patient-reported outcomes measure comprised of three correlated scales (Applied Cognition, Daily Activities, and Mobility). All items from each scale are Likert type, so that a respondent chooses a response from an ordered set of four response options. The most appropriate item (...) class='Hi'>response theory model for analyzing and scoring these items is the multidimensional graded response model (MGRM). During the field testing of the items, an interviewer read each item to a patient and recorded, on a tablet computer, the patient’s responses and the software recorded RTs. Due to the large item bank with over 300 items, data collection was conducted in four batches with a common set of anchor items to link the scale. Van der Linden’s (2007) hierarchical modeling framework was adopted. Several models, with or without interviewer as a covariate and with or without interaction between interviewer and items, were compared for each batch of data. It was found that the model with the interaction between interviewer and item, when the interaction effect was constrained to be proportional, fit the data best. Therefore, the final hierarchical model with lognormal model for RT and the MGRM for response data was fitted to all batches of data via a concurrent calibration. Evaluation of parameter estimates revealed that (1) adding response time information did not affect the item parameter estimates and their standard errors significantly; (2) adding response time information helped reduce the standard error of patients’ multidimensional latent trait estimates, but adding interviewer as a covariate did not result in further improvement. Implications of the findings for follow up adaptive test delivery design are discussed. (shrink)

Students can use an educational system’s help in unexpected ways. For example, they may bypass abstract hints in search of a concrete solution. This behavior has traditionally been labeled as a form of gaming or help abuse. We propose that some examples of this behavior are not abusive and that bottom-out hints can act as worked examples. We create a model for distinguishing good student use of bottom-out hints from bad student use of bottom-out hints by means of logged (...) class='Hi'>response times. We show that this model not only predicts learning, but captures behaviors related to self-explanation. (shrink)

The central nervous system uses feedback processes that occur at multiple time scales to control interactions with the environment. The long-latency response is the fastest process that directly involves cortical areas, with a motoneuron response measurable 50 ms following an imposed limb displacement. Several behavioral factors concerning perturbation mechanics and the active role of muscles prior or during the perturbation can modulate the long-latency response amplitude in the upper limbs, but the interactions among many of (...) these factors had not been systematically studied before. We conducted a behavioral study on thirteen healthy individuals to determine the effect and interaction of four behavioral factors – background muscle torque, perturbation direction, perturbation velocity, and task instruction – on the LLRa evoked from the flexor carpi radialis and extensor carpi ulnaris muscles after velocity-controlled wrist displacements. The effects of the four factors were quantified using both a 0D statistical analysis on the average perturbation-evoked EMG signal in the period corresponding to an LLR, and using a timeseries analysis of EMG signals. All factors significantly modulated LLRa, and their combination nonlinearly contributed to modulating the LLRa. Specifically, all the three-way interaction terms that could be computed without including the interaction between instruction and velocity significantly modulated the LLR. Analysis of the three-way interaction terms of the 0D model indicated that for the ECU muscle, the LLRa evoked when subjects are asked to maintain their muscle activation in response to the perturbations was greater than the one observed when subjects yielded to the perturbations, but this effect was not measured for muscles undergoing shortening or in absence of background muscle activation. Moreover, higher perturbation velocity increased the LLRa evoked from the stretched muscle in presence of a background torque, but no effects of velocity were measured in absence of background torque. Also, our analysis identified significant modulations of LLRa in muscles shortened by the perturbation, including an interaction between torque and velocity, and an effect of both torque and velocity. The time-series analysis indicated the significance of additional transient effects in the LLR region for muscles undergoing shortening. (shrink)

Following a hypnotic amnesia suggestion, highly hypnotically suggestible subjects may experience amnesia for events. Is there a failure to retrieve the material concerned from autobiographical memory, or is it retrieved but blocked from consciousness? Highly hypnotically suggestible subjects produced free-associates to a list of concrete nouns. They were then given an amnesia suggestion for that episode followed by another free association list, which included 15 critical words that had been previously presented. If episodic retrieval for the first trial had been (...) blocked, the responses on the second trial should still have been at least as fast as for the first trial. With semantic priming, they should be faster. In fact, they were on average half a second slower. This suggests that the material had been retrieved but blocked from consciousness. A goal-oriented information processing framework is outlined to interpret these and related data. (shrink)

Peer review poses many challenges for journals. A downside of high rejection rates and sometimes delayed responses in publication decision by journals is a long time period between original submission of a manuscript and its ultimate acceptance and publication. One way of accelerating the process which might be worth considering is multiple submission. This commentary addresses that issue.

We report on five experiments investigating response choices and response times to simple science questions that evoke student “misconceptions,” and we construct a simple model to explain the patterns of response choices. Physics students were asked to compare a physical quantity represented by the slope, such as speed, on simple physics graphs. We found that response times of incorrect answers, resulting from comparing heights, were faster than response times of correct answers comparing slopes. This result (...) alone might be explained by the fact that height was typically processed faster than slope for this kind of task, which we confirmed in a separate experiment. However, we hypothesize that the difference in response time is an indicator of the cause of the response choice. To support this, we found that imposing a 3-s delay in responding increased the number of students comparing slopes on the task. Additionally a significant proportion of students recognized the correct written rule , but on the graph task they incorrectly compared heights. Finally, training either with repetitive examples or providing a general rule both improved scores, but only repetitive examples had a large effect on response times, thus providing evidence of dual paths or processes to a solution. Considering models of heuristics, information accumulation models, and models relevant to the Stroop effect, we construct a simple relative processing time model that could be viewed as a kind of fluency heuristic. The results suggest that misconception-like patterns of answers to some science questions commonly found on tests may be explained in part by automatic processes that involve the relative processing time of considered dimensions and a priority to answer quickly. (shrink)

In this paper I argue that new attempts to undermine the principle of alternative possibilities via appeal to time travel fail. My argument targets a version of a Frankfurt-style counterexample to the principle recently developed by Spencer. I argue that in avoiding one prominent objection to standard Frankfurt-style counterexamples Spencer’s time travel case runs afoul of another. Furthermore, the very feature of the case which makes it initially appealing also makes it impossible to revise the case such that (...) it can avoid this further objection. Thus Spencer’s time travel case and cases like it provide no dialectical advantages over other Frankfurt-style cases. I conclude that this strategy for refining such cases should be abandoned. (shrink)

The investigation of how humans perceive and respond to emotional signals conveyed by the human body has been for a long time secondary compared with the investigation of facial expressions and emotional scenes recognition. The aims of this behavioral study were to assess the ability to process emotional body postures and to test whether motor response is mainly driven by the emotional content of the picture or if it is influenced by motor resonance. Emotional body postures and scenes (...) (IAPS) divided into three clusters (fear, happiness, and neutral) were shown to 25 healthy subjects (13 males, mean age ± SD: 22.3 ± 1.8 years) in a three-alternative forced choice task. Subjects were asked to recognize the emotional content of the pictures by pressing one of three keys as fast as possible in order to estimate response times (RTs). The rating of valence and arousal was also performed. We found shorter RTs for fearful body postures as compared with happy and neutral postures. In contrast, no differences across emotional categories were found for the IAPS stimuli. Analysis on valence and arousal and the subsequent item analysis showed an excellent reliability of the two sets of images used in the experiment. Our results show that fearful body postures are rapidly recognized and processed, probably thanks to the automatic activation of a series of central nervous system structures orchestrating the defensive threat reactions, strengthening and supporting previous neurophysiological and behavioral findings in body language processing. (shrink)

The goal of the present study was to evaluate the roles of response times in the achievement of students in the following latent ability domains: verbal, math and spatial reasoning, mental flexibility, and scientific and mechanical reasoning. Participants were 869 students who took on the Multiple Mental Aptitude Scale. A mixture item response model was implemented to evaluate the roles of response times in performance by modeling ability and non-ability classes. Results after applying this model to the (...) data across domains indicated the presence of several behaviors related to rapid responding which were covaried with low achievement likely representing unsuccessful guessing attempts. (shrink)