Talks and Conference Presentations
Cognitive Neuroscience Society
9th Annual Meeting
April 14-16, 2002
San Francisco, California
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Brain Activation Evoked by the Perception of Gaze Shifts: Influences of Timing and Context
Kevin Pelphrey1,4, Jeremy Goldstein1, J. Steven Reznick3, Joseph Piven4, Truett Allison2, Gregory McCarthy1
The role of lateral temporal-parietal cortical regions including the superior temporal sulcus (STS) in the perception of eye movements is an important issue in cognitive neuroscience. We conducted an fMRI experiment at 4T to investigate the degree to which activity in regions involved in the perception of shifts in gaze could be modulated by varying the context and timing of an observed gaze shift. In five conditions, an animated character was presented from the shoulders up with eyes forward. A box flickered within the character’s FOV. In two conditions, the figure gazed toward the box with a 1- or 3-sec SOA. In two other conditions, the character gazed towards an empty location with a 1- or 3-sec SOA. In the final condition, the character’s eyes did not move. We examined effects of timing (1 vs. 3 seconds), context (correct vs. incorrect), and motion (gaze shift vs. no gaze shift) on the hemodynamic response (HDR) in STS. Significant STS activations were associated with the conditions involving observed eye movements. This activity occurred primarily in the right hemisphere. Activity was greatest in the crux of the STS, where the main body and ascending branch of the STS join. HDR peaks were later in the conditions involving 3-sec SOAs than in those involving 1-sec SOAs. Activity was greater in response to incorrect gaze shifts than correct shifts. This study extends previous research by demonstrating that the timing and context of an observed socially relevant gaze shift influence the HDR characteristics in STS.
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Working Memory Load Dissociates Transient and Delay Activity in Normal Aging
B.A. Townsend1, A.P. Jha2, C. Lustig3, J.R. Petrella1, P.M. Doraiswamy1, G. McCarthy1
We examined the consequences of varying memory load on activity within prefrontal, intraparietal, and fusiform cortex. Fourteen older adults (ages 59-77) performed a delayed-response face working memory task where each trial consisted of a 3s memory set (S1) of either 1 or 3 faces, a 15s delay period, and a single-face probe (S2) to which subjects made a motor response indicating whether this face was part of the memory set. The long delay interval was employed to disentangle transient evoked responses to S1 and S2 stimuli from delay interval activity. Within each anatomically-defined region of interest, analysis epochs were time-locked to S1, with fMRI percent signal change defined relative to a prestimulus baseline. S1-evoked amplitude responses within middle frontal gyrus (MFG), intraparietal sulcus (IPS), and fusiform gyrus (FFG) were greater for 3 faces compared to 1 face S1-events. Differences between the 3 and 1 face conditions were greatest early in the delay period, peaking at 9s. Transient S1 responses within MFG and IPS were followed by significant delay activity that was load-invariant. FFG activity returned to baseline prior to S2. S2 responses were observed within MFG, IPS, and FFG, but this activation was uninfluenced by memory load. Similar to previous results in young subjects reported by Jha and McCarthy (2000), these results show that memory load has its strongest effect early in the delay interval, peaking at 9s and declining prior to S2.
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The Effects of Stimulus Duration upon Visual Cortical Activation: Evidence from Functional MRI and Intracranial ERPs
Scott A. Huettel1, Martin J. McKeown1, Sarah Hart1, Truett Allison2, Allen W. Song1, and Gregory McCarthy1
We investigated the relation between neuronal activity measured by intracranial event-related potentials (ERPs) and the hemodynamic response (HDR) measured by functional magnetic resonance imaging (fMRI) for visual stimuli of different durations. Stimuli were 100ms, 500ms, or 1500ms duration static checkerboards presented singly at fixation in random order. Intracranial ERPs were recorded from subdural electrodes in striate and extrastriate visual cortices of patients being monitored for epileptic seizures. Initial short-latency ERPs (90-200 ms) from calcarine and fusiform cortices did not differ across stimulus durations. However, large-amplitude slow waves persisting over stimulus duration were prominent in peri-calcarine electrodes. These sustained ERPs terminated in large transient offset ERPs. The sustained and offset ERPs were small or absent at fusiform electrodes. For comparison, healthy young adults were scanned using fMRI at 4T with TR of 500ms using identical stimuli. Activation foci were identified in calcarine cortex, fusiform cortex, and an anterior region corresponding to V5. In peri-calcarine regions, HDR amplitude increased dramatically with increasing stimulus duration: the HDR amplitude was approximately 5 times greater for the 1500ms duration than for the 100ms duration. We conclude that sustained neural activity, not the discrete neuronal events reflected by short-latency ERPs, determines the relative amplitude of the HDR in peri-calcarine regions across stimulus durations. We further note that stimulus duration has different effects upon HDR amplitude in different brain regions. In regions corresponding to V5, for example, HDR amplitude was smaller than in peri-calcarine cortex and was similar in amplitude across all stimulus durations.
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Adult Age Differences in the Hemodynamic Response during Visual Target Detection Measured by Functional MRI
David J. Madden1, Scott A. Huettel1,3, Wythe L. Whiting1, Niko J. Harlan1, Linda K. Langley1, James M. Provenzale2, and Gregory McCarthy2,3
We investigated age-related changes in the properties of the hemodynamic response (HDR) measured by functional magnetic resonance imaging (fMRI) during performance of a visual target detection (oddball) task. Participants made the same button-press response both to frequently occurring standard items (colored squares, 87% of trials) and to infrequently occurring novel items (photographs of objects, 6% of trials). Target items (colored circles, 7% of trials) required a different button-press response. FMRI images were acquired at 1.5T using a spiral gradient-echo pulse sequence with TR of 1000ms. Behavioral responses of the older adults (60 – 70 years) were slower than those of the younger adults (19 – 25 years), particularly to the novel items. For both age groups, target items led to activation in the middle frontal gyrus, insular, and motor cortex, whereas novel items elicited activation in the fusiform and inferior frontal gyri. Although peak HDR amplitude was generally similar between age groups, an age-related decline in the spatial extent of activation was evident. We also observed a delay in the onset of the older adults’ HDR, for both the signal amplitude and signal-noise ratio data. This delay was associated with activation for both the target and novel items, but was particularly pronounced for the fusiform activation in response to novel items, suggesting a visual complexity effect. We conclude that dependent measures in fMRI studies, including the HDR waveform, signal-noise ratio, and spatial extent of activation, may vary in cognitive tasks as a result of normal aging.
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