In unanesthetized guinea pigs, thalamic (CGM), and cortical (auditory I) neurons were recorded simultaneously. Nine of 69 neuron pairs showed a positive cross-correlation of their spontaneous activities, with increased discharge probability of the cortical neuron beginning 2--5 ms after the discharge of the CGM-neuron. The individual neurons of such pairs had an identical CF and the same spectral responsiveness. The responses of cortical neurons to pure tones were much more phasic than those of the corresponding CGM-neurons. Thalamic neurons could be driven up to much higher AM- and FM-modulation frequencies (100 Hz) than cortical neurons, which usually ceased to follow AM-frequencies above 20 Hz. Stronger or weaker suppression of tonic response components in cortical and thalamic neurons and the lower AM-range of cortical neurons is related to stronger or weaker intracortical and intrathalamic inhibition respectively. Response characteristics to FM-stimuli are similar to those of AM-stimuli. All CGM and cortical neurons responded to a variety of natural calls of the same or of other species. Responses of CGM-cells represented more components of a call than cortical cells even if the two cells were synaptically connected. In cortical cells, repetitive elements of a call were not represented if the repetition rate was too high. High modulation frequencies within a call, such as those of the fundamental frequency, could still be separated in the response of some CGM-neurons, but never in those of cortical neurons. Both CGM and cortical cells responded essentially to transients (amplitude or frequency modulations) within a call, if spectral components of such elements were within the spectral sensitivity of the cell. Spectral components outside the spectral sensitivity range could result in suppression of spontaneous discharge rate. Responses of cortical and CGM-cells, and thus the representation of call elements by neuronal responses, varied with the intensity of a call. It is suggested that, at higher levels of the auditory system, essential information about the temporal features of complex sounds may be represented by neural responses to transients in various spectral regions.

1. In the cat visual cortex (VC), electrophoretic glutamate application at a depth corresponding to layer VI may have excitatory or inhibitory effects on relay cells of the lateral geniculate nucleus (LGN). Corticofugal excitation was seen, if the receptive field centers (RFCs) of the VC neurons recorded at the application site were within 2.3 degrees of the RFCs of the LGN neurons under test. Inhibitory effects were seen if the RFCs of both cells were further apart up to 3.1 degrees. Glutamate application at more superficial cortical sites had no effect on LGN-neuron activity. 2. Cross-correlation analysis between spontaneous activities of simultaneously recorded VC and LGN neurons revealed excitatory cortico-geniculate connections in 18 pairs with RFCs separated by less than 1.7 degrees. In 15 pairs the peak latency of the excitation was 2--5 msec (3.4 msec in the average), 3 pairs showed long cortico-geniculate latencies (13--18 msec). The existence of a fast and slow cortico-geniculate system is suggested. 3. Inhibitory cortico-geniculate interaction was demonstrated with cross-correlation analysis in 8 pairs of which 4 had RFCs separated by more than 1.7 degrees. The onset latency of the inhibition was 2--7 msec except for 2 pairs with about 20 msec latency. 4. Most of the LGN neurons which were affected by cortical glutamate application or which showed an excitatory or inhibitory connection with a VC neurons were sustained cells, while the majority of VC neurons which were recorded in the effective glutamate application sites or which showed a significant interaction with LGN neurons in the cross-correlogram were binocularly driven and complex, with mostly large RFCs (mean diameter 3.5 degrees). They responded briskly to moving small spots as well as to moving slits. 5. It is concluded that the corticofugal excitatory effect is transmitted through monosynaptic links from VC neurons located in layer VI (complex cell) to LGN relay neurons (mostly sustained-cell) and this system is organized in a precise topographical manner. 6. In an Appendix neuron pairs which showed a positive correlation in the geniculo-cortical direction were described. The findings may support the view that complex as well as simple cells are drive monosynaptically from geniculo-cortical afferents of the sustained or transient type.

The effects of an inhibitor of GABA synthesis, 3-mercaptopropionic acid (MP), and of the GABA antagonist bicuculline (BIC), on the direction and orientation sensitivity of visual cortical neurons were investigated using a computer-controlled stimulus presentation system. Intravenous administration of MP, which was usually more effective than if administered microelectrophoretically, induced a slight, but significant reduction in these properties of about half of the neurons tested. The effect of electrophoretic BIC was in the same direction but clearer than that of MP. In 71% of the simple cells, direction sensitivity was virtually lost during administration of BIC while orientation sensitivity was never completely eliminated in any neuron tested. Simultaneous administration of both drugs (MP systemically, BIC electrophoretically) caused more complete modification of the sensitivities than single administration of each. In four out of thirteen neurons tested, orientation sensitivity was completely abolished. The excitatory receptive fields slightly increased in size and became virtually round. The response magnitude to the optimal stimulus was increased by each drug along and by both. The present results further support the hypothesis that intracortical inhibition plays a major if not an exclusive role for the orientation and direction sensitivity of cortical cells.

Postsynaptic potentials (PSPs) were recorded from cat striate cortical cells by the whole-cell in vivo recording technique using patch-clamp electrodes. EPSPs and IPSPs evoked by flashing bars on the receptive field at different positions and orientations revealed the spatial structure of the excitatory and inhibitory inputs. The elongation of the excitatory input field (length:width ratio) was found to be minimal (mean ratio of 1.7) and much lower than those reported for spike discharges. Two-dimensional receptive field response profiles of early PSPs were recorded by flashing a small spot of light over a square matrix covering the receptive field. These recordings also showed only mild degrees of elongations of the receptive field. Such elongations could be the result of either an excitatory input from the geniculate that is already biased for orientation or an excitatory convergence from a limited number of LGN fields arranged in a row. In most first- order cells, we found that inhibition was contributing significantly to orientation selectivity. Often prominent IPSPs could be evoked by stimuli of nonoptimum orientations. Presence of inhibition could also be inferred by the way that EPSPs were sharply cut off by inhibition. When the amplitude of an EPSP was measured at different latencies after its onset, the EPSP was found to be very broadly tuned to orientation at the beginning, but showing increasing orientation selectivity with time. It is proposed that this progressive development of orientation selectivity is due to (1) inhibitory inputs arriving after the first wave of excitation, (2) intracortical excitatory inputs from other cells tuned to similar orientations, and (3) voltage-sensitive mechanisms such as NMDA channels.

The auditory fields in the cortex of the guinea pig were investigated with microelectrode mapping techniques. Pure tones of varying frequencies and amplitudes were used as acoustic stimuli. Mainly, multiunit activity was recorded.A large tonotopic area is found in the anterior half of the auditory cortex. This area is named the anterior field (field A). Frequency tuning curves of multiunits in field A are generally narrow. Responses to tone stimuli are strong, and latencies are short. Low best frequencies are represented rostrally, high best frequencies caudally. The tonotopy is continuous and quite regular. Field A is narrow dorsally and becomes gradually broader ventrally. Correspondingly, the isofrequency lines slightly diverge from dorsal to ventral.Caudal to the first field, there is a second, smaller tonotopic area. It lies in the dorsal half of the posterior auditory cortex and is therefore named the dorsocaudal field (field DC). The frequency specificity of the cell clusters in this area is as strong as in field A, but the tonotopy is discontinuous: In the dorsal half of field DC, high best frequencies (16-32 kHz) are represented rostrally; the low frequencies (0.5-2.8 kHz) are represented immediately caudal to the high frequencies, while the intermediate frequencies are missing. Ventrally in field DC, the frequency representation is more complete. Except for this discontinuous map, we did not notice any differences between fields A and DC. A third tonotopic field was found rostral to field A. This field extends over a surface of less than 1 mm2 and was named the small field (field S). It contains a complete representation of the frequency range; high best frequencies are located rostrally, low frequencies caudally. The response latencies are slightly longer in field S than in fields A or DC, and the tuning curves are broader.A broad strip of nontonotopic cortex (auditory belt) surrounds fields A and DC caudally. We subdivided this area into the dorsocaudal and the ventrocaudal belt region. In both areas, tuning curves are often broad, and response latencies are longer than in the tonotopic cortex. In the dorsocaudal belt, most multiunits react with a phasic on-response to pure tones; in the ventrocaudal belt, tonic responses occur more frequently. Another nontonotopic region is located in the anterior auditory cortex, rostral to the tonotopic fields, and was therefore named the rostral belt. Tuning curves in this area are broad, latencies are short, and reponse thresholds are often high.In the discussion, the guinea pig is compared with other mammalian species. Species-specific features in the organization of the tonotopic cortex of the guinea pig are revealed.