Metacontrast and brightness discrimination

An attempt was made to obtain U-shaped masking functions in two metacontrast experiments. Trained Ss judged whether a square test stimulus (TS) was bright or dim. The TS was presented alone or in conjunction with an adjacent pair of square masking stimuli (MS) whose energy equaled the bright TS. The stimulus onset asynchronies (SOA) rangedfrom 0 to 125 msec. The task minimized the role of apparent movement cues as a reliable basis for judgrnent. Similar studies have employed TS plus MS vs MS alone as the alternatives, allowing apparent movement to be a cue. Brightness accuracy was a U-shaped function of SOA. This finding is consistent with neural-net models (Weisstein, 1968). However, analysis of Ss’ response bias suggested an alternative explanation involving the MS as a comparison stimulus at short SOA. It was concluded that U-shaped masking functions are also consistent with theories based upon independent component processes, e.g., Schurman and Eriksen (1970) and Uttal (1970).     

Effect of apomorphine on retention of a brightness discrimination in dopamine supersensitive rats

Apomorphine is known to improve the retention performance of a brightness discrimination task after post-training intrahippocampal application to rats. The present study shows that the retention performance of dopamine supersensitive rats was improved by a low dose of apomorphine which was still ineffective in control animals. Dopamine supersensitivity by itself had no effect on acquisition and retention parameters of brightness discrimination. The result further supports the assumption that dopamine receptors in rat hippocampus can modulate the formation of memory.     

Laterality, attention and rate effects in the auditory temporal discrimination of chronic schizophrenics: The effect of treatment with chlorpromazine

Schizophrenic patients, non-psychotic patients and healthy subjects were required to detect longer target tones from a sequence of short tones presented to either ear. The rate of stimulus presentation and the frequency of switches between the ears were varied. The schizophrenic patients were examined for four weeks while on standard clinical doses of chlorpromazine, for four weeks while on placebo, and for four weeks following reinstatement of medication. Lateral asymmetries in the performance of the schizophrenics were found to diminish in proportion to both the duration and the dosage of medication. The results are consistent with the hypothesis that chlorpromazine causes an improvement of left hemisphere performance relative to the right. The performance of the schizophrenics, relative to that of comparison subjects, deteriorated as the rate of stimulus presentation increased, supporting the hypothesis that schizophrenics have particular difficulties in response selection or organization.     

The effect of instructions on the judgement of brightness

A brightness constancy experiment using ten subjects showed that different instructions significantly affect the subject's PSE judgement. Variations in the reflectance of the standard target and in the background and illumination of the variable target did not influence the effect of different instructions. The use of the D-ratio as constancy index in this study confirms an earlier theoretical finding that other brightness constancy ratios introduce an artefact into experimental results.     

Direct transfer from a horizontal-vertical discrimination to a brightness discrimination in the rat

Eight rats were trained on a discrimination between horizontal and vertical striations, and subsequently between black and white vertical striations. Subjects originally trained with horizontal positive preferred black to white and subjects trained with vertical positive preferred white to black.

The present note reports briefly on a transfer effect in rats, observed incidentally in an experiment designed for a different purpose. The rats were trained in a jumping stand to discriminate between horizontal and vertical striations, and when training was complete, were tested on a brightness discrimination.     

A diffusion model analysis of the effects of aging on brightness discrimination

The effects of aging on decision time were examined in a brightness discrimination experiment with young and older subjects (ages, 60-75 years). Results showed that older subjects were slightly slower than young subjects but just as accurate. Ratcliff's (1978) diffusion model was fit to the data, and it provided a good account of response times, their distributions, and response accuracy. There was a 50-msec slowing of the nondecision components of response time for older subjects relative to young subjects, but response criteria settings and rates of accumulation of evidence from stimuli were roughly equal for the two groups. These results are contrasted with those obtained from letter discrimination and signal-detection-like tasks.