Spatial Sensitivity in Field PAF of Cat Auditory Cortex

Abstract

We compared the spatial tuning properties of neurons in two fields [primary auditory cortex (A1) and posterior auditory field (PAF)] of cat auditory cortex. Broadband noise bursts of 80-ms duration were presented from loudspeakers throughout 360 degrees in the horizontal plane (azimuth) or 260 degrees in the vertical median plane (elevation). Sound levels varied from 20 to 40 dB above units' thresholds. We recorded neural spike activity simultaneously from 16 sites in field PAF and/or A1 of alpha-chloralose-anesthetized cats. We assessed spatial sensitivity by examining the dependence of spike count and response latency on stimulus location. In addition, we used an artificial neural network (ANN) to assess the information about stimulus location carried by spike patterns of single units and of ensembles of 2-32 units. The results indicate increased spatial sensitivity, more uniform distributions of preferred locations, and greater tolerance to changes in stimulus intensity among PAF units relative to A1 units. Compared to A1 units, PAF units responded at significantly longer latencies, and latencies varied more strongly with stimulus location. ANN analysis revealed significantly greater information transmission by spike patterns of PAF than A1 units, primarily reflecting the information transmitted by latency variation in PAF. Finally, information rates grew more rapidly with the number of units included in neural ensembles for PAF than A1. The latter finding suggests more accurate population coding of space in PAF, made possible by a more diverse population of neural response types.

Specialization for Sound Localization in Fields A1, DZ, and PAF of Cat Auditory Cortex

“…The across-location ranges of first-spike latencies were given by the differences between the longest and shortest mean first-spike latencies across all stimulus locations. Our presentation of latencies is consistent with that in our previous reports (Stecker et al, 2003;2005b) in that latencies of individual units are given by geometric means, whereas the central tendencies of distributions across units is given by medians.…”

“…That is, all units recorded at a DZ array placement were counted as DZ units even if, for instance, one or more of them showed responses more typical of A1 units. As in our previous acute experiments (Stecker et al, 2003;2005b), the recording arrays in field A1 were oriented roughly parallel to cortical anatomical columns, whereas those arrays that were placed in DZ and PAF on the banks of sulci tended to cross cortical columns. Two cats received array placements in both right and left hemispheres, whereas arrays were restricted to the right hemisphere in the other three cats; in the illustrations, stimulus locations are shown as contra-or ipsilateral relative to the side of the recording site.…”

“…Extracellular action potentials (spikes) were identified offline from the stored neural waveforms using custom software based on principal component analysis of spike shape (Furukawa et al, 2000;Stecker et al, 2003). We encountered well-isolated single units and, more often, spikes from multiple unresolved units.…”

“…We adapted methods from our previous studies to quantify the accuracy with which spike firing patterns of individual units and ensembles of units could signal sound-source locations (Furukawa and Middlebrooks, 2001;Mickey and Middlebrooks, 2003;Stecker et al, 2003); the classification scheme used here differed from those in our previous studies in that it used more limited temporal resolution and in that it yielded mean localization errors as a function of target location. Classifications of responses were based on three-element vectors comprising spike counts within onset, long-latency, and offset time windows (defined above).…”

“…Unilateral inactivation of PAF or AES, or combined inactivation of A1 and DZ, reduced localization accuracy to around chance levels (Malhotra et al, 2004), whereas selective inactivation of A1 or DZ produced more limited deficits, which differed between fields (Malhotra et al, 2008). Similarly, physiological recordings in α-chloralose-anesthetized cats revealed differences in spatial sensitivity among neurons in fields A1, DZ, and PAF (Stecker et al, 2003;2005b). In those studies, A1 neurons demonstrated relatively broad spatial sensitivity, with sensitivity broadening even further as sound levels increased.…”