We have examined the activity levels produced in various areas of the human occipital cortex in response to various motion stimuli using functional magnetic resonance imaging (fMRI) methods. In addition to standard luminance-defined (first-order) motion, three types of second-order motion were used. The areas examined were the motion area V5 (MT) and the following areas that were delineated using retinotopic mapping procedures: V1, V2, V3, VP, V3A, and a new area that we refer to as V3B. Area V5 is strongly activated by second-order as well as by first-order motion. This activation is highly motion-specific. Areas V1 and V2 give good responses to all motion stimuli, but the activity seems to be related primarily to the local spatial and temporal structure in the image rather than to motion processing. Area V3 and its ventral counterpart VP also respond well to all our stimuli and show a slightly greater degree of motion specificity than do V1 and V2. Unlike V1 and V2, the response in V3 and VP is significantly greater for second-order motion than for first-order motion. This trend is evident, but less marked, in V3A and V3B and absent in V5. The results are consistent with the hypothesis that first-order motion sensitivity arises in V1, that second-order motion is first represented explicitly in V3 and VP, and that V5 (and perhaps also V3A and V3B) is involved in further processing of motion information, including the integration of motion signals of the two types.

Velocity discrimination thresholds for drifting luminance gratings were measured as a function of the time interval between test and reference gratings, using a two-interval, forced-choice procedure. Discrimination thresholds, expressed as Weber fractions (delta V/V), were independent of interstimulus intervals (ISIs) ranging from 1-30 s, demonstrating perfect short-term retention of velocity information. When a third grating was briefly presented halfway through a 10-s ISI, memory masking was observed. Discrimination thresholds in memory masking were unaffected by maskers of the same velocity but increased by 100% when test and masker velocity differed by a factor of 2. The results are interpreted with reference to a model where the short-term memory for simple stimulus attributes is assumed to be organized in terms of arrays of memory stores linked in a lateral inhibitory network.

The short-term memory for spatial frequency information was assessed by measuring the spatial frequency discrimination thresholds for briefly flashed luminance gratings as a function of the time interval between the test and reference gratings, using a computercontrolled two-interval forccdchoicc procedure. Discrimination thresholds were stable for intentimulus intervals in the range 1-30 sec under all conditions tested. At low contrasts, short exposure times and low spatial frequencies discrimination thresholds increased, but no interactions between stimulus parameten affecting thresholds and intentimulus interval were observed. It is concluded that factors limiting spatial discrimination are associated with the sensory coding stage. Spatial discrimination and visual memory may be based on a common representation, which is perfectly retained in short-term memory. Visual half-field tests revealed no hemispheric differences in the processing and retention of spatial frequency information.

Multiple sclerosis is a debilitating disorder resulting from scattered lesions in the central nervous system. Because of the high variability of the lesion patterns between patients, it is difficult to relate existing biomarkers to symptoms and their progression. The scattered nature of lesions in multiple sclerosis offers itself to be studied through the lens of network analyses. Recent research into multiple sclerosis has taken such a network approach by making use of functional connectivity. In this review, we briefly introduce measures of functional connectivity and how to compute them. We then identify several common observations resulting from this approach: (a) high likelihood of altered connectivity in deep-gray matter regions, (b) decrease of brain modularity, (c) hemispheric asymmetries in connectivity alterations, and (d) correspondence of behavioral symptoms with task-related and task-unrelated networks. We propose incorporating such connectivity analyses into longitudinal studies in order to improve our understanding of the underlying mechanisms affected by multiple sclerosis, which can consequently offer a promising route to individualizing imaging-related biomarkers for multiple sclerosis.

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Neurophysiological studies indicate the existence of an area in the extrastriate monkey cortex specialized for the processing of stimulus motion. The present investigation was conducted to determine whether a homologous area exits in the human cortex that underlies the processing and short-term storage of velocity information. Contrast detection and velocity discrimination thresholds were measured in a group of 23 patients with unilateral focal damage to either the lateral occipital, temporal, or posterior parietal cortex. Their results were compared to those of 23 age-matched control subjects. Detection and discrimination thresholds were determined for spatially truncated sinewave gratings presented 4 degrees eccentric of fixation randomly in either the left and right visual fields. Contrast detection thresholds were measured in a spatial two-alternative forced-choice paradigm for three different drift rates (1, 2, and 4 Hz) for leftward and rightward drift directions. Simultaneous velocity discrimination thresholds were determined for reference and test gratings presented 4 degrees left and right of fixation. Sequential velocity discrimination thresholds were measured using a delay, with a interstimulus interval (ISIs) of 1, 3, and 10 sec. In a subset of five patients with superior temporal lobe damage, spatial frequency discrimination thresholds for stationary gratings were also determined. The results indicate the following: (1) contrast detection thresholds for drifting gratings did not significantly differ between the patient and control groups; (2) velocity discrimination thresholds were significantly elevated in the patients; (3) velocity discrimination thresholds significantly increased with increasing ISI in the patients; (4) velocity discrimination thresholds were elevated most when the patients had a lesion in the superior temporal cortex; (5) in the subgroup of five patients with superior temporal lobe damage, spatial frequency discrimination thresholds were not significantly elevated. The results suggest that there is a visual area in the human posterior temporal cortex that is involved in the processing and short-term storage of the velocity of moving visual stimuli.