Discrimination in the Sense of Flutter: New Psychophysical Measurements in Monkeys

Abstract

Humans and monkeys have similar capacities to discriminate the frequencies of mechanical sinusoids delivered to their hands in the range that corresponds to the sense of flutter . Previous studies showed that monkeys can discriminate whether comparison stimuli are higher or lower in frequency than a base stimulus that does not vary from trial to trial during an experiment. We verified this result in two monkeys trained in this manner. To confirm that these animals were able to discriminate, we tested them in a variant of the task in which the frequency of the base stimulus changed randomly from trial to trial. The monkeys failed to discriminate in this new testing mode; instead they seemed to categorize the comparison stimuli, ignoring the base stimulus. After further training in the randomized base condition, the two monkeys learned to discriminate accurately. We then explored how the stimulation parameters affected performance. We found that animals could discriminate accurately with stimulus durations as short as 250 msec, with interstimulus intervals as long as 10 sec, with 50% differences between base and comparison stimulus amplitudes or when stimulated on a different finger. Performance did not degrade in these conditions, even though the monkeys had never been trained or tested under them. The results show that monkeys may try to categorize rather than discriminate when the task allows either strategy, although they are capable of performing true discriminations very robustly. These findings have important implications for investigating the neuronal processes underlying sensory discrimination. Key words: flutter; psychophysics; discrimination; categorization; monkeys; vibrotactile stimuliAn important problem in sensory physiology is the isolation of the neural codes that explain the capacity of a subject to make detections and discriminations of sensory stimuli. In this respect, LaMotte and Mountcastle (1975) and Mountcastle et al. (1990) have made a number of important observations in a sensory modality called the sense of flutter. They determined that both humans and monkeys have similar capacities for detecting and discriminating the frequencies of mechanical sinusoids delivered to their hands. The aim of those studies was to discover how the neural code of primary somatosensory cortex (S1) is related to somesthetic performance in the sense of flutter. In the task they designed, animals had to indicate whether the frequency of a comparison stimulus was lower or higher than a base stimulus that did not vary in frequency from trial to trial during a run. The results revealed a set of neurons with quickly adapting properties, the activity of which was entrained by the stimuli, firing in phase with the oscillatory signal. These neurons responded identically to the two stimuli, they did so even during passive stimulation, and their activities seemed unrelated to any kind of comparison process that presumably takes place during discrimination. Mountcastle and colleagues (1990) concluded that the neuronal s...

Inhibitory control by an integral feedback signal in prefrontal cortex: A model of discrimination between sequential stimuli

“…Whereas this mechanism uses an integrator to store the memory of stimulus 1, the inhibitory feedback affects the encoding of that stimulus. Importantly, instead of encoding the product of amplitude and time, as occurs with a straightforward integration of the stimulus, integral feedback control leads to delay activity that depends only on the amplitude of the stimulus, so long as a minimal duration is reached (35). Increasing the duration of f1 in our simulations leads to just a small increase in memory activity (see Fig.…”

Neural correlates of a postponed decision report

“…2, gray), which correspond to the two possible behavioral choices that the monkeys have. Note also that three comparison frequencies (18,22, and 26 Hz) can be judged as either higher or lower than f1; for these, the monkeys must rely on the stored f1 percept to discriminate consistently (10,19,22).…”

“…The sensory discrimination task used here has already been described (ref. 22; see also SI). Monkeys were handled according to the institutional standards of the National Institutes of Health and Society for Neuroscience.…”

“…In the task we previously used, monkeys report whether the second stimulus frequency (f2) is higher or lower than the first stimulus frequency (f1) (22). This cognitive operation requires that subjects compare information of f1 temporally stored in working memory to the current information of f2 to form a decision of whether f2 Ͼ f1 or f2 Ͻ f1, and to immediately report the outcome by pressing one of two push buttons.…”

“…The vibrotactile discrimination task used in these studies simulates a behavioral condition in which a perceptual decision based on a sensory evaluation is immediately reported through a voluntary action (22). However, depending on the behavioral demands, a perceptual decision can be postponed for later report.…”