| 1. Effects of aging on response properties of neurons in visual pathway of catsContrast sensitivity, the ability of detecting relative changes in illumination of visual signals, is believed to be a fundamental characteristic of the visual system and the most important measure in spatial vision. Psychophysical studies on human subjects have proven that visual contrast sensitivities of aged individuals reducedsignificantly compared with young adult ones. Previous studies havedemonstrated that this age-related change cannot be completely attributed to the deterioration in optical factors, but seems more relevant to alterations occurred in the central nervous system. Visual information is processed hierarchically. Neurons in visual pathway give graded responses to different levels of stimulus contrast. Retinal ganglion cells (RGC), together with neurons in the dorsal lateral geniculate nucleus (LGN), provide a relatively faithful representation of image contrast. The coding of contrast gain control appears at the primary visual cortex (A17), and the responses of neurons in other visual cortices also exhibit prominent contrast gain control which may be mediated by a divisive inhibitory mechanism. Previous studies on senescent monkeys demonstrate that both MT and V1neurons are less sensitive to contrast than those in young monkeys. Neuronal changes that accompany senescence in some visual areas have been described individually, but remarkably little is known about whether senescence exerts identical adverse effect on the function of visual areas occupying various stages in visual processing. The various areas along visual pathway involve the processing of basic and complex perceptual functions, with more complex functions carried out at progressively higher levels. Psychophysical studies show that age-related degeneration of perception of higher-order stimuli (e.g., contrast-defined second-order patterns) is significantly more pronounced and can be detected much earlier than lower-order tasks (e.g., luminance-defined first-order stimuli). However, the corresponding physiological mechanism of this phenomenon is still unclear. Using extracellular single-unit and multi-unit in vivo recording technique, we compared the contrast responses function of neurons in various stages along visual pathwayof young and old paralyzed, anesthetized cats.We systematically collected and compared the contrast response function properties of neurons along visual pathway, including lateral geniculate nucleus (LGN), primary visual area (A17), secondary visual area (A18), and posteromedial lateral suprasylvian cortex (PMLS) of young and old cats, and analyzed the difference between them. We chose these sites because they represent distinctly different levels within a hierarchy of visual processing.We found that the effects of aging on LGN neurons are negligible. Y cells exhibited smaller CH values than X cells in both young and old groups, while there was no significant difference between X and Y cells. Results demonstrated that CH, maximal evoked response, spontaneous activity and signal-to-noise ratios of LGN neurons in old cats were relatively unchanged compared with young onesHowever, neurons in the three cortical visual areas were all significantly affected by aging. Particularly, the contrast sensitivity of neurons in old cats is significantly declined compared with the young ones. Additionally, deteriorated contrast sensitivity in three cortical areas was accompanied by enhanced maximum visual response, higher levels of spontaneous activity and decreased signal-to-noise ratios, while LGN neurons exhibited largely normal response properties. Our results suggest that there was a progressively greater effect of aging on neurons at successively higher hierarchical stages in the visual pathway. The contrast sensitivity of PMLS neurons had the most severe age-related deterioration, followed by A18and A17, while LGN neurons were relatively unaffected by aging。The current study systematically analyzed how the CRFs of neurons, at four different stages in visual pathway of cats, were affected by aging. We found that:(1) reduced contrast sensitivity of neurons in three cortical areas was accompanied by enhanced maximal visually-evoked responses, increased spontaneous activity, and decreased SNR value; and (2) there was a progressively greater effect of aging on neurons at successively higher hierarchical stages in the visual pathway. LGN neurons were relatively unaffected by aging, while the contrast response properties of neurons in three cortical visual areas were significantly impaired by aging, PMLS neurons had the most severe age-related deterioration, followed by A18and A17. Our findings suggest possible neural mechanisms underlying deficits on both easy and complex visual tasks observed during senescence.2. Acute alcohol exposure on response properties of neurons in visual pathway of catsNormal individuals after acute alcohol ingestion have been demonstrated a diminished ability to perceive changes in the relative velocities of other vehicles and a diminished ability to adjust one’s own vehicle speed accordingly. Psychological studies have demonstrated that motion perception can be impaired by acute alcohol ingestion and subjects exhibited decreased performances at task based on speed perception after acute alcohol ingestion. Performance of eye-movement tasks that rely in part on speed and acceleration estimation also suffered after acute alcohol exposure. These declines could be attributed to some alcohol-related differences in perception of target speed. The impairments in visual cortical functions mediating the deficits in visual motion perception after acute alcohol ingestion have been observed in visual evoked potential study. Indeed, we have previously reported that direction selectivity of cells in primary visual cortex degraded after acute alcohol exposure. However, another essential part of motion perception, speed, has not been studied in detail. The effect of acute alcohol exposure upon the cortical mechanism mediating the perception of visual motion speed in mammalian is still unclear.The posteromedial lateral suprasylvian cortex (PMLS) of cats, like its presumed primate homologue, the middle-temporal (MT) area, is intimately involved in cortical encoding and decoding of speed information about visual motion. Many neurons in PMLS are tuned to the speed of a moving visual stimulus. These cells are thought to play an important role in the analysis of stimulus speed. Lesions in cat PMLS adversely affect the performance on complex motion perception, speed discrimination, local motion signals integration and motion signals extraction from noise. PMLS is also thought to be involved in the global motion analysis to determine heading direction and guide locomotion. In view of the foregoing, any functional abnormality of speed-selective neurons in area PMLS seems to have a potential to affect speed perception of visual motion speed adversely. It is, therefore, tempting to explore the speed tuning properties of PMLS neurons after acute alcohol exposure, which could provide some valuable clues on the physiological mechanisms mediating alcohol-related impairments in speed perception.We used random-dot patterns to study the effects of acute alcohol on speed-tuning curves in cortical area PMLS of cat. Our results provide evidence for a significant degradation of speed response properties in PMLS. Cells after acute alcohol treatment preferred lower speeds than those before. A broadened speed-tuning bandwidth of PMLS cells was also observed after acute alcohol administration. Additionally, response modulation and discrimination capacity for speed after acute alcohol administration was significantly weaker than those before. Concurrently, PMLS cells after acute alcohol exposure showed decreased spontaneous activity, peak responses and signal-to-noise ratios. The foregoing neural changes may contribute to the alcohol-related deficits in visual motion perception observed in behavioral studies. |
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