The neural basis of time perception is unknown. Here we show that neurons in the posterior parietal cortex (area LIP) represent elapsed time relative to a remembered duration. We trained rhesus monkeys to report whether the duration of a test light was longer or shorter than a remembered "standard" (316 or 800 ms) by making an eye movement to one of two choice targets. While timing the test light, the responses of LIP neurons signaled changes in the monkey's perception of elapsed time. The variability of the neural responses explained the monkey's uncertainty about its temporal judgments. Thus, in addition to their role in spatial processing and sensorimotor integration, posterior parietal neurons encode signals related to the perception of time.

The dorsolateral prefrontal cortex plays a critical role in guiding actions that ensue seconds after an instruction. We recorded from neurons in area 46 and the frontal eye field (FEF) while monkeys performed a memory-guided eye movement task. A visual cue signaled whether a small or large liquid reward would accompany a correct response. Many neurons in area 46 responded more when the monkey expected a larger reward. Reward-related enhancement was evident throughout the memory period and was most pronounced when the remembered target appeared in the neuron's response field. Enhancement was not present in the FEF. The mixture of neural signals representing spatial working memory and reward expectation appears to be a distinct feature of area 46.

The magnitude of experienced reward as a function of the pulse frequency and current in trains of fixed duration delivered to the medial forebrain bundle of the rat was measured using a new psychophysical method in which the parameters of the brain stimulation reward on one lever are adjusted to oifset the effect of changing the rate of reward on a competing lever. Subjective reward magnitude is a steep sigmoidal function of both pulse frequency and current. The growth of reward to its half-maximal level was approximated by a power function with an exponent that varied from 2 to 10. Within a subject, the exponent was the same for both current and pulse frequency, which supports the hypothesis that the magnitude of reward from a train of fixed duration is determined by the rate at which action potentials are generated in the population of reward-relevant axons (the counter hypothesis). This rate is proportional to Current x Pulse Frequency.

Primary sensory cortices have been assumed to serve as stimulus analyzers while cognitive functions such as learning and memory have been allocated to “higher” cortical areas. However, the primary auditory cortex (A1) is now known to encode the acquired significance of sound as indicated by associatively-induced specific shifts of tuning to the frequencies of conditioned stimuli (CS) and gains in area of CS representations. Rewarding brain stimulation can be a very powerful motivator and brain reward systems have been implicated in addictive behavior. Therefore, it is possible that a cue for brain reward will gain cortical territory and perhaps thereby increase its control of subsequent behavior. To investigate the effect of brain reward on cortical organization, adult male rats (n = 11) were first tested with varying amounts of stimulation of the ventral tegmental area (VTAstm) to generate sigmoidal psychometric functions of nose poke (NP) rates as a function of reward magnitude (duration). Next, we attempted to accomplish tone control of NPs by maintaining intertrial NPs using a low reward duration and presenting a 20 s tone (2.0 kHz, 70 dB) which signaled an increase in reward to a high magnitude 10 s after tone onset. Tone control was demonstrated by a significant increase in the rate of NPs during the first 10 s of tone presentation, which anticipated the delivery of the high magnitude of reward. Tone control was achieved in seven of 11 subjects. This was accompanied by a highly specific and significant gain in representational area, specifically for the half-octave range centered on the CS frequency. However, this plasticity developed only in tone-controlled (TC) animals. The auditory cortex of non-tone-controlled subjects (n = 4) did not differ from that of naïve controls (n = 9) although their VTAstm was as rewarding as for the TC group. These findings reveal that auditory instrumental behavior can be controlled by rewarding VTAstm and that such control appears necessary for the highly specific recruitment of cortical cells to increase the representation of a sound that acquires behavioral importance.