Elsevier

Neurobiology of Aging

Volume 21, Issue 4, July–August 2000, Pages 577-584
Neurobiology of Aging

Original Articles
Age-related differences in visual perception: a PET study

https://doi.org/10.1016/S0197-4580(00)00144-5Get rights and content

Abstract

To assess age-related differences in cortical activation during form perception, two classes of visual textures were shown to young and older subjects undergoing positron emission tomography (PET). Subjects viewed even textures that were rich in rectangular blocks and extended contours and random textures that lacked these organized form elements. Within-group significant increases in regional cerebral blood flow (rCBF) during even stimulation relative to random stimulation in young subjects were seen in occipital, inferior and medial temporal regions, and cerebellum, and in older subjects, in posterior occipital and frontal regions. Group by texture type interactions revealed significantly smaller rCBF increases in older subjects relative to young in occipital and medial temporal regions. These results indicate that young subjects activate the occipitotemporal pathway during form perception, whereas older subjects activate occipital and frontal regions. The between-group differences suggest that age-related reorganization of cortical activation occur during early visual processes in humans.

Introduction

Evidence from recent neuroimaging studies has shown age-related differences in the brain regions and networks activated during visual stimulation. Age-related differences in cortical activation during visual word identification [29], visual processing of faces and location [18], and working memory for faces [19] have been found using positron emission tomography (PET). In addition, age-related differences using PET have been shown during memory encoding and retrieval of word pairs [6] and during a card-sorting task [32]. Grady et al. [20] has also shown that older subjects demonstrated a markedly different activation pattern during a face recognition task relative to young subjects, especially in the ventral occipital cortical regions. Although these studies provide insight into age-related differences in visual perception that may result from differences in high level visual processes such as complex feature integration required for the perception of complex objects, the present study was designed to address a more basic aspect of visual function. That is, do perceptual differences associated with normal aging begin earlier in visual processing with changes in feature extraction?

The present study investigates the hypothesis that the activation pattern generated by viewing salient form elements differs for young and older healthy subjects. Based on prior functional neuroimaging studies that have shown age-related differences in visual processing, we predict that the two groups will differ in a specific manner, with either decreased occipitotemporal activation and/or additional activation outside of the occipitotemporal pathway with aging.

Experiments in nonhuman primates [44], and lesion [11], [45] and functional neuroimaging studies in humans [17], [22], [30] have shown that there are two distinct visual processing pathways in extrastriate cortex. Anatomical studies have shown projections from striate cortex into occipital extrastriate regions. From these extrastriate regions the projections diverge either into inferior temporal cortex or into the inferior parietal cortex, thus the anatomical basis for the two pathways [46]. The ventral occipitotemporal pathway is used primarily for perception of objects whereas the dorsal occipitoparietal pathway is important for organization of spatial relationships among objects. Many age-related changes are reported to occur in regions along these visual pathways based on structural neuroimaging studies of the aging human brain [10], determinations of neocortical cell counts in normal aging human brain [43], and studies of pyramidal neuronal loss with aging in the hippocampus, the anterior most extent of the ventral visual pathway [2], [49].

Many aspects of visual function deteriorate slightly with age such as acuity [12] and contrast sensitivity [40], [51]. Higher level functions such as those needed for visual search [28], [33], visual attention [13], [26], visual word identification [25], [27], and visuomotor tracking [31], [50] are also impaired in healthy aging. Additionally, visuospatial functions change with age such as processes required for spatial integration [37], localization [39], and mental rotation of visual stimuli [34]. Age-related changes have also been observed in tasks involving perception [1] as well as encoding and recognition of visual stimuli [9], [41]. This age-related slowing of visual function has been interpreted as reduced processing efficiency or effectiveness [37] and may be related to the neuropathological changes associated with aging.

In this study, the ability to perceive structural organization within visual images was assessed using a passive visual stimulation paradigm [5]. To define cortical regions involved in this type of form perception, two classes of achromatic textures were presented to both young and older subjects undergoing PET. The baseline condition involved viewing random textures, which exhibit a random arrangement of black and white pixels. The stimulation condition involved viewing even textures, which exhibit pixels organized into elongated contours and rectangular blocks of a single color. The random and even textures share the same average luminance and spatial frequency content but differ in the organization of the pixels that make up the image. Previous studies of texture perception [24] have shown that humans can readily discriminate between random and even textures based on the differences in these local features. Furthermore, previous imaging studies have shown that the perception of this type of visual form involves the occipitotemporal pathway in young subjects [3], [4], [5]. Assessment of brain activation in the elderly in the present study will provide additional information regarding age-related cortical function along this visual pathway.

Section snippets

Subjects

Twelve healthy young adults (five women, seven men; ages 20–37; mean ± SD = 27.3 ± 6.0 years; 10 right handed, two left handed) and 14 healthy older adults (six women, eight men; ages 51–73; mean ± SD = 62.1 ± 8.5 years; 11 right handed, three left handed) participated in this study. The NIA Institutional Review Board approved all procedures. After full explanation of purpose, procedures and risks of the study, informed consent was obtained from each subject upon enrollment. During screening,

Results

Statistically significant brain areas activated on a within-group basis will be presented first followed by brain areas that showed significantly different activation patterns between groups. In the young subjects, cerebral activation during even texture stimulation relative to random texture stimulation occurred in visual extrastriate regions of the brain. Bilateral activation was observed in the fusiform (BA 19) and lingual gyri (BA 19). Activation of the middle occipital gyrus (BA 18/19) was

Discussion

Isodipole textures have been used to study visual perception in both monkeys and humans. Non-human primate studies [35] have shown that the striate cortex (V1) extracts higher order spatial correlations from visual textures. When monkeys are shown textures such as those used in the present study, even and random texture interchange produces a prominent form specific signal in the visual evoked potential (VEP) recorded epicortically from V1. Multielectrode recordings also show that this form

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    This work is supported by the NIA Intramural Research Program. Keith Purpura is supported by NIH NS36699.

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