Biomedical subjects
Publications and source records attributed to Vincent Walsh.
Whether the neuronal encoding of number is linear or logarithmic divides cognitive neuroscientists working on mathematical cognition. Recordings from the prefrontal cortex of the monkey support the logarithmic hypothesis. Similarities between number and the coding of other quantities are also beginning to become apparent.
We examined the involvement of visual area V1 in visual detection to assess the role of feedback connections to V1 as proposed in reverse hierarchy theory (Ahissar and Hochstein 2000). In Experiment 1, signal detection was decreased in feature and conjunction detection tasks by repetitive pulse TMS (rTMS) over V1 for 500 ms after stimulus onset. In Experiment 2, rTMS was delayed to allow uninterrupted signal processing for 100 ms after visual stimulus onset. TMS for the subsequent 500 ms now disrupted detection of conjunction but not feature targets. In Experiment 3 we applied double pulse TMS at varying intervals to assess the timing of V1 involvement in these tasks. Single feature detection involved V1 only at some point between 40 and 100 ms after visual array onset; detection of targets defined by conjunctions of features involved V1 throughout the first 100 ms and also between 200 and 240 ms after visual stimulus onset. We suggest that the early effects in the conjunction task are due to repeated sampling of the visual array to extract the signal from external noise. The later effects in conjunction search are attributed to the return projections from secondary visual areas back to V1, consistent with the reverse hierarchy theory. The effects in both tasks are consistent with early and repeated iterations of feed forward and feedback loops as hypothesised in recent neurophysiological experiments (see Foxe and Simpson 2002).
The split-fovea theory proposes that visual word recognition is mediated by the splitting of the foveal image, with letters to the left of fixation projected to the right hemisphere (RH) and letters to the right of fixation projected to the left hemisphere (LH). We applied repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex during a lexical decision task to investigate the extent to which word recognition processes could be accounted for according to the split-fovea theory. Unilateral rTMS significantly impaired lexical decision latencies to centrally presented words, supporting the suggestion that foveal representation of words is split between the cerebral hemispheres rather than bilateral. Behaviorally, we showed that words that have many orthographic neighbors sharing the same initial letters ("lead neighbors") facilitated lexical decision more than words with few lead neighbors. This effect did not apply to end neighbors (orthographic neighbors sharing the same final letters). Crucially, rTMS over the RH impaired lead-, but not end-neighborhood facilitation. The results support the split-fovea theory, where the RH has primacy in representing lead neighbors of a written word.
Whether the contribution of the superior parietal cortex (BA7) to attention-demanding tasks is strictly spatial in nature remains unresolved. We used functional magnetic resonance imaging to explore the behavioural and neuroanatomical correlates of non-spatial search for a conjunction of features within a stream of temporally-distracting stimuli. In addition, we compared these data to those from a conventional visuo-spatial search task, performed by the same subjects, in order to determine the specificity of right BA7 activation. Mode of stimulus-distribution (spatial versus temporal) and search type (target defined by a single feature or a conjunction of features) were manipulated in a 2 x 2 factorial design. Behaviourally, the temporal conjunction task was shown to index temporal selective attention. Accuracy of detecting a second target varied with the temporal proximity of two successive targets when subjects searched for a conjunction of features, but not a single feature. The temporal conjunction task activated a network of areas including right superior parietal cortex and bilateral regions of intraparietal sulcus, frontal operculum and putamen. The two latter regions were selectively activated by the attentional demands of the temporal conjunction task when compared directly to the attentional demands of the spatial conjunction task, implicating these regions specifically in selective attention among temporally-distracting stimuli. By comparison, only a very medial region of right BA7 was selectively activated by the spatial conjunction task. The more lateral region of BA7 previously reported by other groups was engaged to a similar degree by both spatial and temporal versions of the conjunction search task.
Fifty years ago the neurologist MacDonald Critchley observed that parietal cortex damage impaired temporal as well as spatial experience. Whereas the physiological understanding of space has since advanced, the same cannot be said of time. However, in a novel study, recording from single neurons in the macaque, Leon and Shadlen show that a region of the parietal cortex appears to encode time. The area in which these neurons reside also contains spatially selective neurons and overlaps with the area recently reported to contain number neurons.
Recent physiological recording studies in monkeys have suggested that the frontal eye fields (FEFs) are involved in visual scene analysis even when eye movement commands are not required. We examined this proposed function of the human frontal eye fields during performance of visual search tasks in which difficulty was matched and eye movements were neither necessary nor required. Magnetic stimulation over FEF modulated performance on a conjunction search task and a simple feature search task in which the target was unpredictable from trial to trial, primarily by increasing false alarm responses. Simple feature search with a predictable target was not affected. The results establish that human FEFs are critical to visual selection, regardless of the need to generate a saccade command.
The perception of time can be illusory: we have all waited anxiously for important seconds to tick away slowly at the end of a football game and have experienced the truth of the adage "time flies when you're having fun." One illusion of time experience that has recently been investigated, the apparent slowing of the movement of the second hand on the clock when one first looks at it, has been termed "chronostasis," and it has been suggested that the effect is unique to vision and is dependent on eye movements. We sought to test whether the effect is really unique to vision or whether it can also be produced with auditory stimuli. Subjects were asked to judge the length of a silent gap between two tones presented through headphones. When the tones were presented to one ear, subjects judged the duration of the gap veridically. When subjects were required to shift concentration from one ear to the other, however, the judgement of time showed that the auditory system is also susceptible to chronostasis. We suggest that this generalization of chronostasis to another sensory system is consistent with theories of time perception that emphasize a single, multimodal clock for duration estimation rather than a mechanism that is dependent on motor acts.
Recent evidence from individuals born with a profound musical impairment suggests that the ability to process pitch information is normally present from birth. This finding supports the idea that the perception and appreciation of music, both of which critically depend on pitch processing, have a biological basis in the brain.
A visual image can be described by its temporal as well as spatial properties. A recent study of the asynchronous colour and motion perception has led to a new view of perceptual synchronisation, in which the temporal structures of events in the external world provide a general explanation of how events are bound or misbound in time.
Eye movements produce a temporary loss of visual sensitivity known as saccadic suppression, and a distortion of space perception known as saccadic compression. A new study has reported a seemingly related phenomenon --chronostasis---in which one's perception of time also undergoes an illusory distortion during rapid movements of the eyes.
We have always known that some form of clock is needed to measure time. It now seems that a variety of different neural clocks are involved in determining our temporal perceptions, some specialised for shorter and some for longer durations.
Localized repetitive transcranial magnetic stimulation was used to disrupt visuospatial perception in the near and far space of six healthy volunteer subjects. In addition to the baseline condition, they were stimulated over the right posterior parietal cortex, the right or left dorsal occipital cortex or the right ventral occipital cortex, during the brief presentation of a transected horizontal line. Subjects had to indicate whether the part of the line to the left or right of the transection appeared longer. The stimulus display was back-projected on a screen at a viewing distance of either 50 or 150 cm ("near" and "far" space, respectively). Reaction times and choices were measured. In a forced-choice paradigm, subjects showed "pseudoneglect", the natural tendency of neurologically intact subjects to perceive the left side of a centrally transected line as slightly longer than the right. These errors occurred more for lines in near space than for lines in far space. Magnetic stimulation of the right posterior parietal cortex or the right ventral occipital lobe selectively induced a significant shift to the right in the perceived midpoint for near- and far-space lines, respectively. The results reproduced in normal subjects the dissociation between neglect in near and far space that has been described in patients with different right-hemisphere lesions. This dissociation supports the contention that there is a dorsal/near space-ventral/far space segregation of processing in the visual system which reflects the behavioural goals of the two putative visual streams.
Presentation of supraliminal or subliminal visual stimuli that can (or cannot) be detected or identified can improve the probability of the same stimulus being detected over a subsequent period of seconds, hours or longer. The locus and nature of this perceptual priming effect was examined, using suprathreshold stimuli, in subjects who received repetitive pulse transcranial magnetic stimulation over the posterior occipital cortex, the extrastriate motion area V5/MT or the right posterior parietal cortex during the intertrial interval of a visual motion direction discrimination task. Perceptual priming observed in a control condition was abolished when area V5/MT was stimulated but was not affected by magnetic stimulation over striate or parietal sites. The effect of transcranial magnetic stimulation (TMS) on priming was specific to site (V5/MT) and to task - colour priming was unaffected by TMS over V5/MT. The results parallel, in the motion domain, recent demonstrations of the importance of macaque areas V4 and TEO for priming in the colour and form domains.