Long Term Memory for Noise: Evidence of Robust Encoding of Very Short Temporal Acoustic Patterns.

Recent research has demonstrated that humans are able to implicitly encode and retain repeating patterns in meaningless auditory noise. Our study aimed at testing the robustness of long-term implicit recognition memory for these learned patterns. Participants performed a cyclic/non-cyclic discrimination task, during which they were presented with either 1-s cyclic noises (CNs) (the two halves of the noise were identical) or 1-s plain random noises (Ns). Among CNs and Ns presented once, target CNs were implicitly presented multiple times within a block, and implicit recognition of these target CNs was tested 4 weeks later using a similar cyclic/non-cyclic discrimination task. Furthermore, robustness of implicit recognition memory was tested by presenting participants with looped (shifting the origin) and scrambled (chopping sounds into 10- and 20-ms bits before shuffling) versions of the target CNs. We found that participants had robust implicit recognition memory for learned noise patterns after 4 weeks, right from the first presentation. Additionally, this memory was remarkably resistant to acoustic transformations, such as looping and scrambling of the sounds. Finally, implicit recognition of sounds was dependent on participant's discrimination performance during learning. Our findings suggest that meaningless temporal features as short as 10 ms can be implicitly stored in long-term auditory memory. Moreover, successful encoding and storage of such fine features may vary between participants, possibly depending on individual attention and auditory discrimination abilities. Significance Statement Meaningless auditory patterns could be implicitly encoded and stored in long-term memory.Acoustic transformations of learned meaningless patterns could be implicitly recognized after 4 weeks.Implicit long-term memories can be formed for meaningless auditory features as short as 10 ms.Successful encoding and long-term implicit recognition of meaningless patterns may strongly depend on individual attention and auditory discrimination abilities.

Seeing the eyes in acquired prosopagnosia.

Case reports have suggested that perception of the eye region may be impaired more than that of other facial regions in acquired prosopagnosia. However, it is unclear how frequently this occurs, whether such impairments are specific to a certain anatomic subtype of prosopagnosia, and whether these impairments are related to changes in the scanning of faces. We studied a large cohort of 11 subjects with this rare disorder, who had a variety of occipitotemporal or anterior temporal lesions, both unilateral and bilateral. Lesions were characterized by functional and structural imaging. Subjects performed a perceptual discrimination test in which they had to discriminate changes in feature position, shape, or external contour. Test conditions were manipulated to stress focused or divided attention across the whole face. In a second experiment we recorded eye movements while subjects performed a face memory task. We found that greater impairment for eye processing was more typical of subjects with occipitotemporal lesions than those with anterior temporal lesions. This eye selectivity was evident for both eye position and shape, with no evidence of an upper/lower difference for external contour. A greater impairment for eye processing was more apparent under attentionally more demanding conditions. Despite these perceptual deficits, most subjects showed a normal tendency to scan the eyes more than the mouth. We conclude that occipitotemporal lesions are associated with a partially selective processing loss for eye information and that this deficit may be linked to loss of the right fusiform face area, which has been shown to have activity patterns that emphasize the eye region.

The temporal dynamics of the distractor in the global effect.

In the global effect, saccades are displaced towards a distractor if the latter is near to the target, an effect thought to reflect spatial averaging in neurons of the superior colliculus. The temporal dynamics of the global effect have not been well studied, however. We had twelve subjects perform horizontal saccades to a target in trials in which there were either no distractor or a distractor stimulus located 20° above or below the target. The distractor appeared either simultaneously with the target or preceded it by an interval of between 100 and 800 ms, and was either flashed for only 100 ms or remained visible until the subject responded with a saccade. Both flashed and persistent distractors reduced saccadic latency if they preceded target onset, indicating that subjects could use this cue to prepare saccades in advance. Saccadic endpoint was displaced towards a flashed distractor only if it was simultaneous with the target. However, persistent distractors produced a global effect for both simultaneous presentation and distractor-target intervals of 100 ms, but not for longer intervals. We conclude that the global effect requires of the distractor both a recent onset and persistence of the distractor, and that distractor-related activity decays rapidly within 300 ms.

Decisions under risk in Parkinson's disease: preserved evaluation of probability and magnitude.

INTRODUCTION: Unmedicated Parkinson's disease patients tend to be risk-averse while dopaminergic treatment causes a tendency to take risks. While dopamine agonists may result in clinically apparent impulse control disorders, treatment with levodopa also causes shift in behaviour associated with an enhanced response to rewards. Two important determinants in decision-making are how subjects perceive the magnitude and probability of outcomes. Our objective was to determine if patients with Parkinson's disease on or off levodopa showed differences in their perception of value when making decisions under risk. METHODS: The Vancouver Gambling task presents subjects with a choice between one prospect with larger outcome and a second with higher probability. Eighteen age-matched controls and eighteen patients with Parkinson's disease before and after levodopa were tested. In the Gain Phase subjects chose between one prospect with higher probability and another with larger reward to maximize their gains. In the Loss Phase, subjects played to minimize their losses. RESULTS: Patients with Parkinson's disease, on or off levodopa, were similar to controls when evaluating gains. However, in the Loss Phase before levodopa, they were more likely to avoid the prospect with lower probability but larger loss, as indicated by the steeper slope of their group psychometric function (t(24) = 2.21, p = 0.04). Modelling with prospect theory suggested that this was attributable to a 28% overestimation of the magnitude of loss, rather than an altered perception of its probability. CONCLUSION: While pre-medicated patients with Parkinson's disease show risk-aversion for large losses, patients on levodopa have normal perception of magnitude and probability for both loss and gain. The finding of accurate and normally biased decisions under risk in medicated patients with PD is important because it indicates that, if there is indeed anomalous risk-seeking behaviour in such a cohort, it may derive from abnormalities in components of decision making that are separate from evaluations of size and probability.

The global effect for antisaccades.

In the global effect, prosaccades are deviated to a position intermediate between two targets or between a distractor and a target, which may reflect spatial averaging in a map encoded by the superior colliculus. Antisaccades differ from prosaccades in that they dissociate the locations of the stimulus and goal and generate weaker collicular activity. We used these antisaccade properties to determine whether the global effect was generated in stimulus or goal computations, and whether the global effect would be larger for antisaccades, as predicted by collicular averaging. In the first two experiments, human subjects performed antisaccades while distractors were placed in the vicinity of either the stimulus or the saccadic goal. Global effects occurred only for goal-related and not for stimulus-related distractors, indicating that this effect emerges from interactions with motor representations. In the last experiment, subjects performed prosaccades and antisaccades with and without goal-related distractors. When the results were adjusted for differences in response latency, the global effect for rapid responses was three to four times larger for antisaccades than for prosaccades. Finally, we compared our findings with predictions from collicular models, to quantitatively test the spatial averaging hypothesis: we found that our results were consistent with the predictions of a collicular model. We conclude that the antisaccade global effect shows properties compatible with spatial averaging in collicular maps and likely originates in layers with neural activity related to goal rather than stimulus representations.

Response selection in prosaccades, antisaccades, and other volitional saccades.

Saccades made to the opposite side of a visual stimulus (antisaccades) and to central cues (simple volitional saccades) both require active response selection but whether the mechanisms of response selection differ between these tasks is unclear. Response selection can be assessed by increasing the number of response alternatives: this leads to increased reaction times when response selection is more demanding. We compared the reaction times of prosaccades, antisaccades, saccades cued by a central arrow, and saccades cued by a central number, in blocks of either two or six possible responses. In the two-response blocks, reaction times were fastest for prosaccades and antisaccades, and slowest for arrow-cued and number-cued saccades. Increasing response alternatives from two to six caused a paradoxical reduction in reaction times of prosaccades, had no effect on arrow-cued saccades, and led to a large increase in reaction times of number-cued saccades. For antisaccade reaction times, the effect of increasing response alternatives was intermediate, greater than that for arrow-cued saccades but less than that for number-cued saccades. We suggest that this pattern of results may reflect two components of saccadic processing: (a) response triggering, which is more rapid with a peripheral stimulus as in the prosaccade and antisaccade tasks and (b) response selection, which is more demanding for the antisaccade and number-cued saccade tasks, and more automatic when there is direct stimulus-response mapping as with prosaccades, or over-learned symbols as with arrow-cued saccades.

Sensitivity and bias in decision-making under risk: evaluating the perception of reward, its probability and value.

BACKGROUND: There are few clinical tools that assess decision-making under risk. Tests that characterize sensitivity and bias in decisions between prospects varying in magnitude and probability of gain may provide insights in conditions with anomalous reward-related behaviour. OBJECTIVE: We designed a simple test of how subjects integrate information about the magnitude and the probability of reward, which can determine discriminative thresholds and choice bias in decisions under risk. DESIGN/METHODS: Twenty subjects were required to choose between two explicitly described prospects, one with higher probability but lower magnitude of reward than the other, with the difference in expected value between the two prospects varying from 3 to 23%. RESULTS: Subjects showed a mean threshold sensitivity of 43% difference in expected value. Regarding choice bias, there was a 'risk premium' of 38%, indicating a tendency to choose higher probability over higher reward. An analysis using prospect theory showed that this risk premium is the predicted outcome of hypothesized non-linearities in the subjective perception of reward value and probability. CONCLUSIONS: This simple test provides a robust measure of discriminative value thresholds and biases in decisions under risk. Prospect theory can also make predictions about decisions when subjective perception of reward or probability is anomalous, as may occur in populations with dopaminergic or striatal dysfunction, such as Parkinson's disease and schizophrenia.

Line bisection under an attentional gradient induced by simulated neglect in healthy subjects.

Whether an attentional gradient favouring the ipsilesional side is responsible for the line bisection errors in visual neglect is uncertain. We explored this by using a conjunction-search task on the right side of a computer screen to bias attention while healthy subjects performed line bisection. The first experiment used a probe detection task to confirm that the conjunction-search task created a rightward attentional gradient, as manifest in response times, detection rates, and fixation patterns. In the second experiment subjects performed line bisection with or without a simultaneous conjunction-search task. Fixation patterns in the latter condition were biased rightwards as in visual neglect, and bisection also showed a rightward bias, though modest. A third experiment using the probe detection task again showed that the attentional gradient induced by the conjunction-search task was reduced when subjects also performed line bisection, perhaps explaining the modest effects on bisection bias. Finally, an experiment with briefly viewed pre-bisected lines produced similar results, showing that the small size of the bisection bias was not due to an unlimited view allowing deployment of attentional resources to counteract the conjunction-search task's attentional gradient. These results show that an attentional gradient induced in healthy subjects can produce visual neglect-like visual scanning and a rightward shift of perceived line midpoint, but the modest size of this shift points to limitations of this physiological model in simulating the pathologic effects of visual neglect.

The effect of central (macula) sparing on contralateral line bisection bias: a study with virtual hemianopia.

Hemianopic patients show a contralesional bisection bias, but it is unclear whether this is a consequence of their field loss or related to extrastriate damage. One observation cited against the former is that hemianopic bisection bias does not vary with the degree of central (macular) sparing; however, it is unclear to what extent central sparing should affect this bias. Our goal was to determine the effect of central sparing on line bisection biases from field loss alone, with two approaches. First, we studied 12 healthy subjects viewing lines under conditions of virtual hemianopia, created by a gaze-contingent technique. Second, we calculated the effect predicted by a visuospatial model of the effect of central magnification on line representations in the visual system. Our results first replicated the contralateral line bisection bias with hemianopia, confirming that this can be generated by visual hemifield loss in the absence of extrastriate damage. Central sparing had only a modest effect on hemianopic bisection bias, with only slightly less bias with 10° compared to 2° of central sparing. In accordance with these empiric data, computing the center of mass for line representations in our model showed only a shallow decline in bisection bias as central sparing increased from 0 to 10°. We conclude that contralateral bisection bias only decreases slightly with central sparing, and that the absence of a statistically significant effect of central sparing in patients cannot be taken as evidence against a visual origin of contralateral hemianopic line bisection bias.

Line bisection in simulated homonymous hemianopia.

Hemianopic patients make a systematic error in line bisection, showing a contra-lesional bias towards their blind side, which is the opposite of that in hemineglect patients. This error has been attributed variously to the visual field defect, to long-term strategic adaptation, or to independent effects of damage to extrastriate cortex. To determine if hemianopic bisection error can occur without the latter two factors, we studied line bisection in healthy subjects with simulated homonymous hemianopia using a gaze-contingent display, with different line-lengths, and with or without markers at both ends of the lines. Simulated homonymous hemianopia did induce a contra-lesional bisection error and this was associated with increased fixations towards the blind field. This error was found with end-marked lines and was greater with very long lines. In a second experiment we showed that eccentric fixation alone produces a similar bisection error and eliminates the effect of line-end markers. We conclude that a homonymous hemianopic field defect alone is sufficient to induce both a contra-lesional line bisection error and previously described alterations in fixation distribution, and does not require long-term adaptation or extrastriate damage.