Spatial and temporal distribution of odorant-evoked activity in the piriform cortex

Abstract

Despite a remarkably precise spatial representation of odorant stimuli in the early stages of olfactory processing, the projections to the olfactory (piriform) cortex are more diffuse and show characteristics of a combinatorial array, with extensive overlap of afferent inputs and widespread intracortical association connections. Furthermore, although there is increasing evidence for the importance of temporal structure in olfactory bulb odorant-evoked output, little is known about how this temporal patterning is translated within cortical neural ensembles. The present study used multichannel electrode arrays and paired single-unit recordings in rat anterior piriform cortex to test several predictions regarding ensemble coding in this system. The results indicate that odorants evoke activity in a spatially scattered ensemble of anterior piriform cortex neurons, and the ensemble activity includes a rich temporal structure. The most pronounced discrimination between different odorants by cortical ensembles occurs during the first inhalation of a 2 s stimulus. The distributed spatial and temporal structure of cortical activity is present at both global and local scales, with neighboring single units contributing to coding of different odorants and active at different phases of the respiratory cycle. Finally, cross-correlogram analyses suggest that cortical unit activity reflects not only afferent input from the olfactory bulb but also intrinsic activity within the intracortical association fiber system. These results provide direct evidence for predictions stemming from anatomical- and theoretical-based models of piriform cortex.

Figure 1.

A, Electrode arrays were constructed of six to eight 35 μm insulated microwires arranged in two parallel rows and 250 μm inter-electrode distances. This is an image of the array placed at the cortical surface. B, Recording sites were confirmed to be within layer II/III of anterior piriform cortex histologically. Arrows indicate the location of two electrode tips. LOT, Lateral olfactory tract; rf, rhinal fissure. C, Example of multiunit activity recorded from the electrode array and simultaneously recorded respiration (chest wall movement).

Figure 2.

Individual recording sites respond to multiple odorants. Representative raster displays of multiunit activity recorded with electrode arrays are shown. Recordings from single electrodes in four different rats are shown. Each recording site responded to a unique combination of odorants, and responses varied in both magnitude of firing rate and in temporal structure.

Figure 3.

Individual odorants activate multiple recording sites. Representative raster displays of activity from six recording sites in a single rat to three different odorant stimuli are shown. The bottom right shows the relative spatial distribution of the recording sites to each other and shows that individual sites could be >1 mm apart within an animal.

Figure 4.

Individual odorants evoked unit activity distributed throughout the anterior piriform cortex. The images represent a view of the piriform cortex from the dorsal surface of the brain, with rostral to the left and the lateral edge of the brain to the top of each image. Electrode sites are plotted in the bottom in gray. White blocks signify that electrodes from two rats were located in that space. Color panels, Pseudocolor representations of multiunit activity at 59 recording sites plotted as a function of rostrocaudal and mediolateral location of the electrode site. Color represents cumulative activity in a 200 ms time bin over 10 repeats of the stimulus. Time is milliseconds from odorant onset (seconds stimulus duration). Both odorants evoked widespread activation of the anterior piriform cortex, although activity at specific recording sites varied with time.

Figure 5.

Different odorants evoked different ensembles of activity within the anterior piriform cortex. Pairwise statistical comparisons of activity (200 ms bin width) at each recording site evoked by two different odorants are shown. Recording locations are green, and sites with significant differences in activity (p < 0.01) are color coded. Significant differences in odorant-evoked activity were distributed throughout the anterior piriform cortex and were most pronounced during early odorant stimulation. Pseudocolor images are oriented as described in Figure 4.

Figure 6.

A, Mean number of recording sites showing a significant difference in activity evoked by two different odorant stimuli (n = 15 different odorant pair combinations; 200 ms bin width). The ensemble activity displayed the greatest difference between odorants during the first inhalation (400 ms). A second peak occurred at odorant offset. Odorant present during the horizontal bar. Mean ± SEM. B, Higher-resolution analysis (50 ms bins) of initial odor response with average respiratory waveform (chest wall movement) across all animals and stimuli displayed for alignment. Odorant onset in all cases occurred at the transition from inhalation to exhalation. An increase in pattern difference emerged nearly as the first inhalation began and peaked within 100 ms. Mean ± 95% confidence limits.

Figure 7.

Neighboring single units did not preferentially fire in-phase with each other over the respiratory cycle. The correlation between the mean phase angle for simultaneously recorded single-unit pairs was 0.20.

Figure 8.

Representative example of a simultaneously recorded single-unit pair in anterior piriform cortex. Units were isolated with principle component analysis and cluster cutting (A), and mean waveforms for the two units are shown in B. Waveforms are color coded for their phase and peristimulus time histograms. C, E, Single-unit activity of each cell as a function of respiratory cycle. In this case, both cells fired at approximately the same phase of the cycle. D, F, Peristimulus time histograms of singe-unit activity in response to odorant stimulation. The blue cell was significantly excited by this odorant, whereas the simultaneously recorded yellow cell did not show a significant change in activity. G, Cross-correlogram of activity in these two units. The blue line is 1 ms binned activity, and the red line is the smoothed (9 ms sliding average) relationship. H, Activity of the two spike trains was shuffled by one respiratory cycle, and the cross-correlogram was again determined. The center peak in this correlogram represents the joint activity evoked simply as a consequence of respiration-evoked activity. I, A plot of the raw cross-correlogram with the shuffled correlogram subtracted. The activity in these two spike trains still demonstrates a significant peak centered around zero, suggesting they both receive a common input.

Figure 9.

Representative example of a simultaneously recorded single-unit pair in the anterior piriform cortex. Details are as described in Figure 8. In this cell pair, the two cells fired out-of-phase with each other over the course of the respiratory cycle (C, E), and neither showed a significant response to odorant (D, F). The cross-correlogram shows a short-latency narrow peak offset from zero, indicative of a potential direct synaptic connection between these two neurons.