Humans and monkeys have similar abilities to discriminate the difference in frequency between two mechanical vibrations applied sequentially to the fingertips. A key component of this sensory task is that the second stimulus is compared with the trace left by the first (base) stimulus, which must involve working memory. Where and how is this trace held in the brain? This question was investigated by recording from single neurons in the prefrontal cortex of monkeys while they performed the somatosensory discrimination task. Here we describe neurons in the inferior convexity of the prefrontal cortex whose discharge rates varied, during the delay period between the two stimuli, as a monotonic function of the base stimulus frequency. We describe this as 'monotonic stimulus encoding', and we suggest that the result may generalize: monotonic stimulus encoding may be the basic representation of one-dimensional sensory stimulus quantities in working memory. Thus we predict that other behavioural tasks that require ordinal comparisons between scalar analogue stimuli would give rise to monotonic responses similar to those reported here.

Perceptual decisions arise from the activity of neurons distributed across brain circuits. But, decoding the mechanisms behind this cognitive operation across brain circuits has long posed a difficult problem. We recorded the neuronal activity of diverse cortical areas, while monkeys performed a vibrotactile discrimination task. We find that the encoding of the stimuli during the stimulus periods, working memory, and comparison periods is widely distributed across cortical areas. Notably, during the comparison and postponed decision report periods the activity of frontal brain circuits encode both the result of the sensory evaluation that corresponds to the monkey's possible choices and past information on which the decision is based. These results suggest that frontal lobe circuits are more engaged in the readout of sensory information from working memory, when it is required to be compared with other sensory inputs, than simply engaged in motor responses during this task.

The ability to discriminate between two sequential stimuli requires evaluation of current sensory information in reference to stored information. Where and how does this evaluation occur? We trained monkeys to compare two mechanical vibrations applied sequentially to the fingertips and to report which of the two had the higher frequency. We recorded single neurons in secondary somatosensory cortex (S2) while the monkeys performed the task. During the first stimulus period, the firing rate of S2 neurons encoded the stimulus frequency. During the second stimulus period, however, some S2 neurons did not merely encode the stimulus frequency. The responses of these neurons were a function of both the remembered (first) and current (second) stimulus. Moreover, a few hundred milliseconds after the presentation of the second stimulus, these responses were correlated with the monkey's decision. This suggests that some S2 neurons may combine past and present sensory information for decision-making.

We recorded from single neurons of the primary auditory cortex (A1), while trained monkeys reported a decision based on the comparison of 2 acoustic flutter stimuli. Crucially, to form the decision, monkeys had to compare the second stimulus rate to the memory trace of the first stimulus rate. We found that the responses of A1 neurons encode stimulus rates both through their periodicity and through their firing rates during the stimulation periods, but not during the working memory and decision components of this task. Neurometric thresholds based on firing rate were very similar to the monkey's discrimination thresholds, whereas neurometric thresholds based on periodicity were lower than the experimental thresholds. Thus, an observer could solve this task with a precision similar to that of the monkey based only on the firing rates evoked by the stimuli. These results suggest that the A1 is exclusively associated with the sensory and not with the cognitive components of this task.decision making ͉ monkeys ͉ working memory ͉ psychophysics ͉ neurophysiology T he problem of how sensory experiences arise from activity in the brain has stimulated a large amount of research in neuroscience (1, 2). A major component of this problem involves understanding how the brain represents sensory features-that is, what attributes of the neural responses evoked by a stimulus are meaningful for sensation, perception, memory, and decisionmaking? To confront these issues unambiguously, experimental methods should conform to 2 essential conditions: first, the sensory stimulus must be under precise, quantitative control, and second, the subject's psychophysical responses should be well controlled and quantitatively measured. Most experimental paradigms meeting these standards have involved vision (1, 3) and somatosensation (2, 4). In comparison, the amount of research about audition using this approach is scant (5). Most of the studies in the auditory cortex and related areas have described the response properties of neurons to auditory scenes in anesthetized and awake animals, and how these are affected by different task conditions (5-21). But how the neural representations of acoustic stimuli are related to perception, memory and decision making is not known.We addressed this problem by recording the activity of single neurons in the primary auditory cortex (A1), while trained monkeys discriminated the difference in rate of 2 acoustic flutter stimuli (range of 4-40 Hz). The sensation of acoustic flutter is produced by slow repetition of an acoustic stimulus (11). The rate of the acoustic flutter is determined by the interval between the acoustic stimuli (pulse trains). In the acoustic flutter discrimination task, monkeys report whether the second stimulus rate (f2) is higher or lower than the first stimulus rate (f1). This cognitive operation requires that subjects compare information of f2 with a stored trace of f1 to form a decision, that is, whether f2 Ͼ f1 or f2 Ͻ f1, and to report their perceptual evaluation after a short, fix...

Recent studies have reported that sensory cortices process more than one sensory modality, challenging the long-lasting concept that they process only one. However, both the identity of these multimodal responses and whether they contribute to perceptual judgments is unclear. We recorded from single neurons in somatosensory cortices and primary auditory cortex while trained monkeys discriminated, on interleaved trials, either between two tactile flutter stimuli or between two acoustic flutter stimuli, and during discrimination sets that combined these two sensory modalities. We found neurons in these sensory cortices that responded to stimuli that are not of their principal sensory modality during these tasks. However, the identity of the stimulus could only be decoded from responses to their principal sensory modality during the stimulation periods and not during the processing steps that link sensation and decision making. These results suggest that multimodal encoding and perceptual judgments in these tasks occur outside the sensory cortices studied here.