Conditioned tone control of brain reward behavior produces highly specific representational gain in the primary auditory cortex.

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 20s tone (2.0kHz, 70dB) which signaled an increase in reward to a high magnitude 10s after tone onset. Tone control was demonstrated by a significant increase in the rate of NPs during the first 10s 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.

Representation of time by neurons in the posterior parietal cortex of the macaque.

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.

Avoidance learning facilitates temporal processing in the primary auditory cortex.

The primary auditory cortex is now known to be involved in learning and memory, as well as auditory perception. For example, spectral tuning often shifts toward or to the frequency of the conditioned stimulus during associative learning. As previous research has focused on tonal frequency, less is known about how learning might alter temporal parameters of response in the auditory cortex. This study addressed the effects of learning on the fidelity of temporal processing. Adult male rats were trained to avoid shock that was signaled by an 8.0 kHz tone. A novel control group received non-contingent tone and shock with shock probability decreasing over days to match the reduced number of shocks received by the avoidance group as they mastered the task. An untrained (nai ve) group served as a baseline. Following training, neuronal responses to white noise and a broad spectrum of tones were determined across the primary auditory cortex in a terminal experiment with subjects under general anesthesia. Avoidance conditioning significantly improved the precision of spike-timing: the coefficient of variation of 1st spike latency was significantly reduced in avoidance animals compared to controls and nai ves, both for tones and for noise. Additionally, avoidance learning was accompanied by a reduction of the latency of peak response, by 2.0-2.5 ms relative to nai ves and approximately 1.0 ms relative to controls. The shock-matched controls also exhibited significantly shorter peak latency of response than nai ves, demonstrating the importance of this non-avoidance control. Plasticity of temporal processing showed no evidence of frequency specificity and developed independently of the non-temporal parameters magnitude of response, frequency tuning and neural threshold, none of which were facilitated. The facilitation of temporal processing suggests that avoidance learning may increase synaptic strength either within the auditory cortex, in the subcortical auditory system, or both.

The effect of scopolamine on matching behavior and the estimation of relative reward magnitude.

We investigated the behavioral effects of scopolamine on rats that bar pressed for trains of electrically stimulating pulses under concurrent variable interval schedules of reward. For the first half of the session (30 min) a 1:4 ratio in the programmed number of stimulation trains delivered at each option was in effect. At the start of the second half of the session, an unsignaled reversal in the relative train number (4:1) occurred. We tracked the relative magnitude of reward estimated for each contiguous pair of reinforced visits to competing options. Scopolamine hydrobromide led to a reduction in the relative magnitude of reward. A similar result was obtained in a follow-up test in which relative magnitude was manipulated by varying the pulse frequency of stimulation, while equating the train number at each option. The effect of scopolamine hydrobromide could not be attributed to undermatching, side bias, nor to an effect of scopolamine on the reward integration process. When the same rats were treated with scopolamine methylbromide, no effects on matching behavior were observed. Our results suggest a cholinergic basis for the computation of choice variables related to matching behavior. (PsycINFO Database Record

CS-specific modifications of auditory evoked potentials in the behaviorally conditioned rat.

The current report provides a detailed analysis of the changes in the first two components of the auditory evoked potential (AEP) that accompany associative learning. AEPs were recorded from the primary auditory cortex before and after training sessions. Experimental subjects underwent one (n=5) or two (n=7) days of conditioning in which a tone, serving as a conditioned stimulus (CS), was paired with mild foot shock. Control subjects received one (n=5) or two (n=7) days of exposure to the same stimuli delivered randomly. Only animals receiving paired CS-US training developed a conditioned tachycardia response to the tone. Our analyses demonstrated that both early components of the AEP recorded from the granular layer of the cortex undergo CS-specific associative changes: (1) the first, negative component (occurring ∼21ms following tone onset) was significantly augmented after one and two days of training while maintaining its latency, and (2) the second, positive component (occurring ∼50ms following tone onset) was augmented after two days of training, and showed a significant reduction in latency after one and two days of training. We view these changes as evidence of increased cortical synchronization, thereby lending new insight into the temporal dynamics of neural network activity related to auditory learning.