Visual discrimination performance

Three male squirrel monkeys previously trained to perform visual discrimination or visual acuity threshold tests were exposed continuously for 90 d to PGDN at a concentration of 262 mg/m3 (approximately 37 ppm) (Jones et al. 1972). The animals were removed from the exposure chambers for a 2-h period once a week for the respective behavior tests. A fourth trained monkey exposed to filtered room air under the same conditions served as the control. The only sign during exposure was mydriasis (excessive dilatation of the pupil of the eye), which increased from slight to moderate. There were no changes in avoidance behavior in the monkeys as determined by the visual tests. 

On a task of visual discrimination - the Karni-Sagi task, for example - individuals improve their performance without knowing how or why (as they learn the task) after sleep they then do better... 

Evenden JL, Marston HM, Jones GH, Giardini V, Lenard L, Everitt BJ, Robbins TW (1989) Effects of excitotoxic lesions of the substantia innominata, ventral and dorsal globus pallidus on visual discrimination, acquisition, performance and reversal in the rat. Behav Brain Res 32 129-149. 

Studies with human infants find that at even one or two days of age, they are able to perform detailed visual discriminations, and they show preferences for visually complex or novel stimuli. While this line of research cannot prove the ability is not learned, it does lend support to these abilities being present at birth in some form. 

Delayed alternation is another possible training protocol for the T/Y-maze, in which animals are rewarded for choosing any goal box in trial one. They are then returned to the start box and released after an inter-trial interval. In trial two, they have to enter the arm not visited in trial one (non-match) and are rewarded. Typically, animals acquire a criterion of 80% correct responses after a short training period. When tested in the presence of A THC, there was a significant drop in performance coupled with a reduction in monoamine turnover in their prefrontal cortex (Jentsch et al. 1997). Animals treated with a similar dose (5 mg/kg) A THC, however, were not impaired in brightness discrimination (Jentsch et al. 1996) or visual discrimination of forms procedures (Mishima et al. 2001) administered in the same apparatus. 

Newborn children from mothers who ate oontaminated fish were more likely to exhibit hypoactive reflexes, more motor immaturity, poorer lability of states, and greater amount of startle. Testing at 4 years of age found that prenatal exposure was associated with poorer performance on the Verbal and the Memory scales of the McCarthy Scales of Children s Abilities, as well as less efficient visual discrimination processing and more errors in short-term memory scanning. Evaluation of the children at 11 years of age showed that prenatal exposure was significantly associated with lower full-scale and verbal IQ scores and poorer reading word comprehension. 

In the raty dizocilpine (75 Jig/kg) had no effect on the acquisition of a spatial discrimination task in a Y-maze, but disrupted reversal learning (Cross et al. 1995). Both the acquisition and reversal of a visual discrimination task were impaired following dizocilpine (75 ig/kg). Dizocilpine (40 ig/kg) also disrupted performance of a five-choice visual reaction time task. 

In addition to these 35 main psychological test variables, a subsidiary set of ten other measures was examined. The standard deviation of each subject s mean reaction time (RT) over several trials was calculated on each of the ten RT measures from the Visual Discrimination, Symbol-Digit Coding and Two-choice Reaction Time tasks. These ten subsidiary measures were included to test the secondary hypothesis that exposure to solvents results primarily in variability in behavior and performance rather than a decrease in the level of performance (cf. Knave, Olson, Elofsson et al, 1978). 

Mohr B, Pulvermuller F, Zaidel E (1994) Lexical decision after left, right and bilateral presentation of function words, content words and non-words evidence for interhemispheric interaction. Neuropsychologia 32(1) 105-124 Moukhles H, Amalric M, Nieoullon A, Daszuta A (1994) Behavioural recovery of rats grafted with dopamine cells after partial striatal dopaminergic depletion in a conditioned reaction-time task. Neuroscience 63(1) 73-84 Muir JL, Everitt BJ, Robbins TW (1996) The cerebral cortex of the rat and visual attentional function dissociable effects of mediofrontal, cingulate, anterior dorsolateral, and parietal cortex lesions on a five-choice serial reaction time task. Cereb Cortex 6(3) 470-481 Muir JL, Bussey TJ, Everitt BJ, Robbins TW (1996) Dissociable effects of AMPA-induced lesions of the vertical limb diagonal band of Broca on performance of the 5-choice serial reaction task and on acquisition of a conditional visual discrimination. Behav Brain Res 82(1) 31-44... 

Fig. 8. Performance on a visual discrimination by a monkey poisoned by methylmercuiy (top) and corresponding blood levels (bottom). Filled triangles represent priming doseS open triangles represent maintenance doses. Bright and dim refer to luminance of visual stimuli and correspond to photopic and scotopic function, respectively.

Winneke et al (1982b) carried out an interesting study which demonstrates how relevant the nature of the learning task is. In their study rats were dosed pre- and post-natally with lead. Animals were tested on a two-way active avoidance task, and later on a visual discrimination task. In the discrimination task the performance of lead-dosed animals was inferior to the controls. In the avoidance task the performance of animals improved significantly in a dose-related fashion, with increasing lead level. 

The results are inconsistent with those of the previous study, to the extent that there was no significant difference in the visual discrimination learning between the groups. In the spatial learning task there were significant differences in the number of days required to reach criterion, with the performance of both maternally and permanently exposed groups inferior to that of controls. 

When retested the performance of both lead-dosed groups was inferior to that of controls, on both the tests. The performance of the maternally dosed animals resembled that of the permanently dosed animals despite the fact that their blood and brain levels were comparable to those of the controls. This implies that early damage is irreversible, and that further exposure does not exacerbate this effect. The study also highlights the functional difference between learning and memory, as there was no difference in the learning of the visual discrimination task, but there were differences in the retention of this skill. 

Experimental exposure studies have attempted to associate various neurological effects in humans with specific trichloroethylene exposure levels. Voluntary exposures of 1 hours resulted in complaints of drowsiness at 27 ppm and headache at 81 ppm (Nomiyama and Nomiyama 1977). These are very low exposure levels, but the results are questionable because of the use of only three test subjects per dose, lack of statistical analysis, sporadic occurrence of the effects, lack of clear dose-response relationships, and discrepancies between the text and summary table in the report. Therefore, this study is not presented in Table 2-1. No effects on visual perception, two-point discrimination, blood pressure, pulse rate, or respiration rate were observed at any vapor concentration in this study. Other neurobehavioral tests were not performed, and the subjects were not evaluated following exposure. 

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