Johan Lauwereyns

A neural correlate of response bias in monkey caudate nucleus

Primates are equipped with neural circuits in the prefrontal cortex, the parietal cortex and the basal ganglia that predict the availability of reward during the performance of behavioural tasks. It is not known, however, how reward value is incorporated in the control of action. Here we identify neurons in the monkey caudate nucleus that create a spatially selective response bias depending on the expected gain. In behavioural tasks, the monkey had to make a visually guided eye movement in every trial, but was rewarded for a correct response in only half of the trials. Reward availability was predictable on the basis of the spatial position of the visual target. We found that caudate neurons change their discharge rate systematically, even before the appearance of the visual target, and usually fire more when the contralateral position is associated with reward. Strong anticipatory activity of neurons with a contralateral preference is associated with decreased latency for eye movements in the contralateral direction. We conclude that this neuronal mechanism creates an advance bias that favours a spatial response when it is associated with a high reward value.

Irrelevant digits affect feature-based attention depending on the overlap of neural circuits

Feature-based attention was investigated by examining the effect of irrelevant information on the processing of relevant information. In all experiments, irrelevant information consisted of digits whose semantic information is known to be processed in parietal areas. Between experiments we varied the degree of parietal involvement in the processing of the relevant feature. The influence of the irrelevant digit on the binary manual response task on the relevant feature was measured by the SNARC effect, a spatial numerical association of response codes demonstrating faster left than right hand responses for small numbers and faster right than left hand responses for large numbers. When processing of the relevant feature depended on parietal cortex, as is the case for orientation processing (exps. 1 and 4), there was an effect of the digits semantic value on response times. Conversely, there was no effect of the irrelevant digit on the processing of color (exps. 2 and 3) or shape (exp. 5), which rely only minimally on parietal resources. After ruling out alternative explanations we conclude that the efficiency of feature-based attention is determined by the degree of neural overlap of structures dedicated to process relevant and irrelevant information.

Feature-Based Anticipation of Cues that Predict Reward in Monkey Caudate Nucleus

A subset of caudate neurons fires before cues that instruct the monkey what he should do. To test the hypothesis that the anticipatory activity of such neurons depends on the context of stimulus-reward mapping, we examined their activity while the monkeys performed a memory-guided saccade task in which either the position or the color of a cue indicated presence or absence of reward. Some neurons showed anticipatory activity only when a particular position was associated with reward, while others fired selectively for color-reward associations. The functional segregation suggests that caudate neurons participate in feature-based anticipation of visual information that predicts reward. This neuronal code influences the general activity level in response to visual features without improving the quality of visual discrimination.

Semantic Influences on Feature-Based Attention Due to Overlap of Neural Circuits

SEMANTIC INFLUENCES ON FEATURE-BASED ATTENTION DUE TO OVERLAP OF NEURAL CIRCUITS Jan Lammertyn1, Wim Fias1 and Johan Lauwereyns2 (1Department of Experimental Psychology, Ghent University, Belgium and 2Laboratory of Sensorimotor Research, National Eye Institute, National Institutes of Health, Bethesda, Maryland, USA)

Interference from irrelevant features on visual discrimination by macaques (macaca fuscata) : A behavioral analogue of the human stroop effect

To study the operation of selective attention in a conflict situation with automatic processes, we trained 4 Japanese macaques (Macaca fuscata) extensively on a manual go/no-go task. The monkey had to discriminate either the color, shape, motion direction, or location of a visual stimulus. In each trial, the behavioral meaning of the relevant feature (go or no-go) could either be congruent or incongruent with irrelevant features of the same stimulus. Reaction times were slowed, and error rates increased when irrelevant stimulus features were incongruent with the required response. The effects were obtained when the monkey attended to the color, shape, or motion direction, but not when it attended to the location of the stimulus. The effects were cumulative so that the interference from 1 incongruent feature was smaller than that from 2 incongruent features. We propose that the present paradigm provides a behavioral analogue of the human Stroop effect.

Exogenous/Endogenous Control of Space-based/ Object-based Attention: Four Types of Visual Selection?

Attention can be directed to an object or to a location, and in a bottom-up or in a top-down fashion. The two dimensions of visual attention have elicited separate lines of research that bypassed the possibility of interactions. In the present paper, the relation between the two dimensions is explored from the initial assumption that they refer to independent aspects. If so, four types of visual selection should appear: exogenous/endogenous control of space-based/object-based attention. Examples of three of the four types are readily found in the literature, but expecting an object does not seem to affect visual selection. Therefore, endogenous control of objectbased attention may not be possible, suggesting that the two dimensions of visual attention are interdependent and require a more integrative theoretical framework. However, research to date may not have provided adequate tests to observe benefits from expecting objects. Three issues remain open: the representation from which object-based selection...

The role of eye movements in decision making and the prospect of exposure effects.

The aim of the current study was to follow on from previous findings that eye movements can have a causal influence on preference formation. Shimojo et al. (2003) previously found that faces that were presented for a longer duration in a two alternative forced choice task were more likely to be judged as more attractive. This effect only occurred when an eye movement was made towards the faces (with no effect when faces were centrally presented). The current study replicated Shimojo et al.s (2003) design, whilst controlling for potential inter-stimuli interference in central presentations. As per previous findings, when eye movements were made towards the stimuli, faces that were presented for longer durations were preferred. However, faces that were centrally presented (thus not requiring an eye movement) were also preferred in the current study. The presence of an exposure duration effect for centrally presented faces casts doubt on the necessity of the eye movement in this decision making process and has implications for decision theories that place an emphasis on the role of eye movements in decision making.

Theta phase shift in spike timing and modulation of gamma oscillation: A dynamic code for spatial alternation during fixation in rat hippocampal area CA1

Although hippocampus is thought to perform various memory-related functions, little is known about the underlying dynamics of neural activity during a preparatory stage before a spatial choice. Here we focus on neural activity that reflects a memory-based code for spatial alternation, independent of current sensory and motor parameters. We recorded multiple single units and local field potentials in the stratum pyramidale of dorsal hippocampal area CA1 while rats performed a delayed spatial-alternation task. This task includes a 1-s fixation in a nose-poke port between selecting alternating reward sites and so provides time-locked enter-and-leave events. At the single-unit level, we concentrated on neurons that were specifically active during the 1-s fixation period, when the rat was ready and waiting for a cue to pursue the task. These neurons showed selective activity as a function of the alternation sequence. We observed a marked shift in the phase timing of the neuronal spikes relative to the theta oscillation, from the theta peak at the beginning of fixation to the theta trough at the end of fixation. The gamma-band local field potential also changed during the fixation period: the high-gamma power (60-90 Hz) decreased and the low-gamma power (30-45 Hz) increased toward the end. These two gamma components were observed at different phases of the ongoing theta oscillation. Taken together, our data suggest a switch in the type of information processing through the fixation period, from externally cued to internally generated.

The effect of semantic information on saccade trajectory deviations

In recent years, many studies have explored the conditions in which irrelevant visual distractors affect saccades trajectories. These previous studies mainly focused on the low-level stimulus characteristics and how they affect the magnitude of curvature. The present study explored the possible effect of high level semantic information on saccade curvature. Semantic saliency was manipulated by presenting irrelevant peripheral taboo versus neutral cue words in a spatial cuing paradigm that allowed for the measurement of trajectory deviations. Findings showed larger saccade trajectory deviations away from taboo (versus neutral) cue words when making a saccade towards another location. This indicates that due to their high semantic saliency, more inhibition was necessarily applied to taboo cue locations to effectively suppress their competing as saccade targets.

Attentional capture and hold: the oculomotor correlates of the change detection advantage for faces.

The present study investigated the influence of semantic information on overt attention. Semantic influence on attentional capture and hold mechanisms was explored by measuring oculomotor correlates of the reaction time (RT) and accuracy advantage for faces in the change detection task. We also examined whether the face advantage was due to mandatory processing of faces or an idiosyncratic strategy by participants, by manipulating preknowledge of the object category in which to expect a change. An RT and accuracy advantage was found for detecting changes in faces compared to other objects of less social and biological significance, in the form of greater attentional capture and hold. The faster attentional capture by faces appeared to overcompensate for the longer hold, to produce faster and more accurate manual responses. Preknowledge did not eliminate the face advantage, suggesting that faces receive mandatory processing when competing for attention with stimuli of less sociobiological salience.