Biomedical subjects

J D Mendola

Publications and source records attributed to J D Mendola.

9 recordsLinked to original sources

PubMed2001

Cortical mechanisms specific to explicit visual object recognition.

The cortical mechanisms associated with conscious object recognition were studied using functional magnetic resonance imaging (fMRI). Participants were required to recognize pictures of masked objects that were presented very briefly, randomly and repeatedly. This design yielded a gradual accomplishment of successful recognition. Cortical activity in a ventrotemporal visual region was linearly correlated with perception of object identity. Therefore, although object recognition is rapid, awareness of an object's identity is not a discrete phenomenon but rather associated with gradually increasing cortical activity. Furthermore, the focus of the activity in the temporal cortex shifted anteriorly as subjects reported an increased knowledge regarding identity. The results presented here provide new insights into the processes underlying explicit object recognition, as well as the analysis that takes place immediately before and after recognition is possible.

Adult↗

PubMed1999

The representation of illusory and real contours in human cortical visual areas revealed by functional magnetic resonance imaging.

Illusory contours (perceived edges that exist in the absence of local stimulus borders) demonstrate that perception is an active process, creating features not present in the light patterns striking the retina. Illusory contours are thought to be processed using mechanisms that partially overlap with those of "real" contours, but questions about the neural substrate of these percepts remain. Here, we employed functional magnetic resonance imaging to obtain physiological signals from human visual cortex while subjects viewed different types of contours, both real and illusory. We sampled these signals independently from nine visual areas, each defined by retinotopic or other independent criteria. Using both within- and across-subject analysis, we found evidence for overlapping sites of processing; most areas responded to most types of contours. However, there were distinctive differences in the strength of activity across areas and contour types. Two types of illusory contours differed in the strength of activation of the retinotopic areas, but both types activated crudely retinotopic visual areas, including V3A, V4v, V7, and V8, bilaterally. The extent of activation was largely invariant across a range of stimulus sizes that produce illusory contours perceptually, but it was related to the spatial frequency of displaced-grating stimuli. Finally, there was a striking similarity in the pattern of results for the illusory contour-defined shape and a similar shape defined by stereoscopic depth. These and other results suggest a role in surface perception for this lateral occipital region that includes V3A, V4v, V7, and V8.

Brain Mapping↗

PubMed1999

Visual discrimination after anterior temporal lobectomy in humans.

OBJECTIVE: To determine whether right anterior temporal lobectomy (RTL) results in perceptual deficits, and whether the perception of particular stimulus features (i.e., shape, motion, color) is affected differentially. BACKGROUND: RTL results in abnormal visual discrimination, recognition, and recall of pictorial material that cannot be easily specified verbally, such as designs and faces. It is unclear whether stimuli must be conceptually meaningful to elicit perceptual deficits. METHODS: Tests were constructed to assess a wide spectrum of basic visual discrimination abilities with simple, meaningless stimuli. The performance of nine patients who underwent left temporal lobectomy (LTL) and nine patients who underwent RTL were compared with that of normal control individuals. The mean excision size along lateral cortex was 3.7 cm for the LTL group and 5.6 cm for the RTL group; mean mesial excision size was 5.2 cm for LTL and 4.6 cm for RTL. RESULTS: Basic visual discrimination capacities were demonstrated to be essentially intact after LTL and RTL, except for a mild loss of blue color discrimination after RTL. CONCLUSIONS: There is little evidence that RTL produces perceptual impairments limited to the domain of pattern perception, or generalizable to nonmeaningful stimuli. The perceptual loss after RTL may be largely restricted to extraction of meaning, and related to the disruption of the circuits that connect the outcome of visual analysis to previously stored semantic information.

Adult↗

PubMed1999

Visual discrimination and attention after bilateral temporal-lobe lesions: a case study.

We studied a woman (Case 1) who acquired achromatopsia, prosopagnosia, and memory loss after sustaining bilateral temporal-lobe lesions. Given her symptoms and locus of lesion, the affected area may be related to the monkey visual area IT. In order to examine her deficits, we assessed her basic discrimination capacities in several domains. She performed normally when stimuli differed in contrast, size, or motion. her performance was abnormal for patterned targets, and was markedly impaired when the patterned targets were less prominent than distractors. This impairment decreased with practice. These symptoms partially resemble the deficits that have been found in monkeys with lesions in visual cortical area V4.

Adult↗

PubMed1998

Functional analysis of primary visual cortex (V1) in humans.

Human area V1 offers an excellent opportunity to study, using functional MRI, a range of properties in a specific cortical visual area, whose borders are defined objectively and convergently by retinotopic criteria. The retinotopy in V1 (also known as primary visual cortex, striate cortex, or Brodmann's area 17) was defined in each subject by using both stationary and phase-encoded polar coordinate stimuli. Data from V1 and neighboring retinotopic areas were displayed on flattened cortical maps. In additional tests we revealed the paired cortical representations of the monocular "blind spot." We also activated area V1 preferentially (relative to other extrastriate areas) by presenting radial gratings alternating between 6% and 100% contrast. Finally, we showed evidence for orientation selectivity in V1 by measuring transient functional MRI increases produced at the change in response to gratings of differing orientations. By systematically varying the orientations presented, we were able to measure the bandwidth of the orientation "transients" (45 degrees).

Brain Mapping↗

PubMed1998

The representation of the ipsilateral visual field in human cerebral cortex.

Previous studies of cortical retinotopy focused on influences from the contralateral visual field, because ascending inputs to cortex are known to be crossed. Here, functional magnetic resonance imaging was used to demonstrate and analyze an ipsilateral representation in human visual cortex. Moving stimuli, in a range of ipsilateral visual field locations, revealed activity: (i) along the vertical meridian in retinotopic (presumably lower-tier) areas; and (ii) in two large branches anterior to that, in presumptive higher-tier areas. One branch shares the anterior vertical meridian representation in human V3A, extending superiorly toward parietal cortex. The second branch runs antero-posteriorly along lateral visual cortex, overlying motion-selective area MT. Ipsilateral stimuli sparing the region around the vertical meridian representation also produced signal reductions (perhaps reflecting neural inhibition) in areas showing contralaterally driven retinotopy. Systematic sampling across a range of ipsilateral visual field extents revealed significant increases in ipsilateral activation in V3A and V4v, compared with immediately posterior areas V3 and VP. Finally, comparisons between ipsilateral stimuli of different types but equal retinotopic extent showed clear stimulus specificity, consistent with earlier suggestions of a functional segregation of motion vs. form processing in parietal vs. temporal cortex, respectively.

Brain Mapping↗

PubMed1997

Functional analysis of V3A and related areas in human visual cortex.

Using functional magnetic resonance imaging (fMRI) and cortical unfolding techniques, we analyzed the retinotopy, motion sensitivity, and functional organization of human area V3A. These data were compared with data from additional human cortical visual areas, including V1, V2, V3/VP, V4v, and MT (V5). Human V3A has a retinotopy that is similar to that reported previously in macaque: (1) it has a distinctive, continuous map of the contralateral hemifield immediately anterior to area V3, including a unique retinotopic representation of the upper visual field in superior occipital cortex; (2) in some cases the V3A foveal representation is displaced from and superior to the confluent foveal representations of V1, V2, V3, and VP; and (3) inferred receptive fields are significantly larger in human V3A, compared with those in more posterior areas such as V1. However, in other aspects human V3A appears quite different from its macaque counterpart: human V3A is relatively motion-selective, whereas human V3 is less so. In macaque, the situation is qualitatively reversed: V3 is reported to be prominently motion-selective, whereas V3A is less so. As in human and macaque MT, the contrast sensitivity appears quite high in human areas V3 and V3A.

Anatomy, Cross-Sectional↗

PubMed1995

Prevalence of visual deficits in Alzheimer's disease.

This study addressed the issue of prevalence and pattern of visual deficits in 77 subjects with Alzheimer's disease (AD) and 111 healthy control subjects. We defined cutoff scores that would be expected from only 1 control subject of 100 (p = 0.01). The percentage of AD subjects who performed at or worse than this level varied across the 16 visual tests from a high of 58% to a low of 0%. The distribution of impairment across tests suggests a high vulnerability in AD of pattern vision, moderate vulnerability of spatial vision, and low vulnerability of motion and flicker perception. We found evidence for heterogeneity in the AD subject group: a subgroup (N = 14) emerged that was characterized by poor performance on the Backward Pattern Masking test, relatively young age, and relatively short duration of AD. Overall, the results indicate that visual dysfunction, especially on Backward Masking, is a common sign of AD.

Aged↗

PubMedDate not supplied

Direction selectivity and physiological compensation in the superior colliculus following removal of areas 17 and 18.

Previous studies indicate that cortical areas 17 and 18 play a prominent role in generating the direction selectivities of neurons in the superior colliculus of the cat. This hypothesis was tested by quantifying the activities of neurons in the superficial collicular layers in intact cats and cats which incurred ablation of areas 17 and 18 and part of area 19. In addition, since behavioral and anatomical studies suggest a functional adjustment in the superior colliculus following removal of inputs from areas 17, 18, and 19 in the neonatal cat, we included a group of neonatally lesioned cats. Computation of an index of directionality indicated that the majority of neurons in intact cats preferred movement in one direction, thus confirming reports of others. Following ablation of areas 17 and 18 and part of area 19 in both groups of lesioned cats, only modest changes in the population indices were detected when poorly responsive neurons were eliminated from the analyses. Based upon levels of visually evoked neuronal activity, our data suggest a physiological compensation by neurons in stratum griseum superficiale following removal of areas 17, 18, and 19 inputs. In the intact and neonatally operated groups, activity in stratum griseum superficiale is high, whereas in the adult lesioned group activity is low. In stratum opticum, neuronal activity was similar in all three groups of cats. These results show that neurons in stratum griseum superficiale undergo a physiological compensation following removal of immature areas 17 and 18.

Adaptation, Physiological↗