Frequency Tuning Of Cells Research Articles

Responses of Midbrain Auditory Neurons in Rats to FM and AM Tones Presented Simultaneously

Speech and other communication signals contain components of frequency and amplitude modulations (FM, AM) that often occur together. Auditory midbrain (or inferior colliculus, IC) is an important center for coding time-varying features of sounds. It remains unclear how IC neurons respond when FM and AM stimuli are both presented. Here we studied IC neurons in the urethane-anesthetized rats when animals were simultaneously stimulated with FM and AM tones. Of 122 units that were sensitive to the dual stimuli, the responses could be grossly divided into two types: one that resembled the respective responses to FM or AM stimuli presented separately ("simple" sensitivity, 45% of units), and another that appeared markedly different from their respective responses to FM or AM tones ("complex" sensitivity, 55%). These types of combinational sensitivities were further correlated with individual cell's frequency tuning pattern (response area) and with their common response pattern to FM and AM sounds. Results suggested that such combinational sensitivity could reflect local synaptic interactions on IC neurons and that the neural mechanisms could underlie more developed sensitivities to acoustic combinations found at the auditory cortex.

Parametric modeling of the temporal dynamics of neuronal responses using connectionist architectures

1. We describe an exploratory approach to the parametric modeling of dynamical (time-varying) neurophysiological data. The models use stimulus data from a window of time to predict the neuronal firing rate at the end of that window. The most successful models were feedforward three-layered networks of input, hidden, and output "nodes" connected by weights that were adjusted during a training phase by the backpropagation algorithm. The memory in these models was represented by delay lines of varying length propagating activation between the layers. Connectionist models with no memory (1 sequential node per layer) as well as zero-memory nonlinear nonconnectionist models were also tested. 2. Models were tested with recordings of neuronal activity from the auditory thalamic nucleus ovoidalis of urethane-anesthetized zebra finches (Taeniopygia guttata). All cells reported here showed phasic/tonic responses. Extensive modeling of one neuron (cell 1) defined a "canonical" architecture, which was most successful in modeling this cell. The canonical model had a zero-memory input layer, a hidden layer with 29 bins representing 185.6 ms, and a single output node whose value as a function of sequential bin position represented the output of the neuron as a function of time. The canonical model achieved convergence on the entire data set for cell 1, including responses to single tone bursts and zebra finch songs. The "frequency" weights of the canonical model matched well excitatory and inhibitory frequencies for cell 1 as determined by the cell's frequency tuning curve. The "memory" weights of the canonical model were dominated by excitation over the first 25 ms followed by inhibition. 3. When trained with only the dynamical responses to tone bursts, the canonical model also accurately predicted the responses to song (average R2 = 0.823). Thus for this neuron the responses to single tone bursts were sufficient to predict most of the responses to six different songs, each presented at three different amplitudes, although a Monte Carlo procedure indicated the residual variance was not just due to noise (P < 0.001). 4. The model's ability to predict the responses to song was further explored by altering the tone burst (training) data. For cell 1, the responses to songs were most strongly related to the phasic/tonic details of the temporal responses to tone bursts. Changes in the characteristic frequency, frequency tuning curves, and rate/intensity function of the neuron had less effect. These experiments would be difficult or impossible to conduct electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of direct current on dc receptor potentials from cochlear inner hair cells in the guinea pig.

Inner hair cell responses to sound were monitored while direct current was applied across the membranous labyrinth in the first turn of the guinea pig cochlea. The current injection electrodes were positioned in the scala vestibuli and on the round window membrane. Positive and negative current (less than 100 microA) caused changes in the sound-evoked dc receptor potentials which were dependent on the sound frequency and intensity. The frequencies most affected by this extracellular current were those comprising the "tip" portion of the inner hair cell frequency tuning characteristic (FTC). The influence of current increased with increasing frequency. Positive current increased the amount of dc receptor potential for the affected frequencies while negative current decreased the potential. Current-induced changes (on a percentage basis) were greater for low intensity sounds and the negative current direction. These frequency specific changes are evidenced as a loss in sensitivity for the tip area of the FTC and a downward shift of the inner hair cell characteristic frequency. Larger current levels (greater than 160 microA) cause more complex changes including unrecoverable loss of cell performance. In separate experiments positive and negative currents (less than 1.1 microA) were injected into the inner hair cell from the recording electrode during simultaneous measurement of the sound-evoked dc receptor potential. This condition caused a shift in IHC sensitivity that was independent of sound frequency and intensity. Positive current decreased the sensitivity of the level of the cell while negative current increased the responses. The effect of current level on sound-evoked dc receptor potential was nonlinear, as comparatively greater increases in cell response were observed for negative than decreases for positive current. The intracellular current injection results are accounted for by the mechano-resistive model of hair cell transduction, where nonlinear responses with current level may reflect outward rectification. Response changes induced by extracellular current are evidence of current effects on both inner and outer hair cells. The frequency and intensity dependences are hypothesized to represent voltage mediated control of inner hair cell response by the outer hair cells.

Dynamic aspects of guinea pig inner hair cell receptor potentials with transient asphyxia.

DC and AC receptor potentials of cochlear inner hair cells in response to tone bursts of various frequencies and intensities were continuously measured during and following periods of transient asphyxia. The effects of asphyxia were most pronounced for low sound pressure level (SPL) acoustic stimuli near the characteristic frequency (CF) of the inner hair cell, leading to vulnerability of the 'tip' of the cell's frequency tuning curve (FTC). The resulting changes in the shape of the FTC are, first, a reduction in tip criterion sensitivity of 10-20 dB without significant loss in sharpness of tuning. Later, when the full effect of 30-45 s asphyxia occurs, tip sensitivity loss between 30 and 65 dB is accompanied by greatly broadened tuning and a shift downward in frequency of the CF by greater than 1/4 octave. The CF shift is due to a progressive loss of high frequency sensitivity. The linear segment of the input-output (intensity) function, plotted as log DC receptor potential versus SPL (at the original CF), becomes longer during the early phase asphyxia, and the slope of the segment declines by 50%. At high SPLs, for all frequencies, the time course of the receptor potential change was similar in shape to that exhibited by the endocochlear potential (EP). In particular, for high sound levels, the recovery of response matches the EP while for low level tip frequency sounds recovery is protracted. No difference between the decline of the AC and DC receptor potentials at CF was observed. Inner hair cell resting membrane potential (Em) hyperpolarized during asphyxia by 2-6 mV, correlating with the change in EP according to a ratio of 1/10 (Em/EP).