Responses Of Midbrain Neurons Research Articles

Sound localization in barn owls studied with manipulated head-related transfer functions: beyond broadband interaural time and level differences

Interaural time and level differences are important cues for sound localization. We wondered whether the broadband information contained in these two cues could fully explain the behavior of barn owls and responses of midbrain neurons in these birds. To tackle this problem, we developed a novel approach based on head-related transfer functions. These filters contain the complete information present at the eardrum. We selected positions in space characterized by equal broadband interaural time and level differences. Stimulation from such positions provides reduced information to the owl. We show that barn owls are able to discriminate between such positions. In many cases, but not all, the owls may have used spectral components of interaural level differences that exceeded the known behavioral resolution and variability for discrimination. Alternatively, the birds may have used template matching. Likewise, neurons in the optic tectum of the barn owl, a nucleus involved in sensorimotor integration, contained more information than is available in the broadband interaural time and level differences. Thus, these data show that more information is available and used by barn owls for sound localization than carried by broadband interaural time and level differences.

Shifting and scaling adaptation to dynamic stimuli in somatosensory cortex

Recent reports have shown that responses of midbrain neurons in the guinea pig rapidly shift the dynamic range of their responses to track changes in the statistics of ongoing sound-level distributions. This results in an increased coding accuracy for the most commonly occurring stimulus intensities. To investigate whether this type of adaptation might also be found in other sensory modalities, we characterized the intensity-response functions of neurons in rat primary somatosensory cortex (S1) to continuous sinusoidal vibration of the whiskers with amplitudes that were changed every 40 ms. Vibration amplitudes were selected randomly such that there was an 80% chance for the amplitude to be drawn from a relatively narrow 'high-probability region' (HPR). Stimulus mean and variance were then manipulated by shifting or widening the HPR. We found that rat S1 neurons adapt to shifts of the HPR mainly by shifting their thresholds, and to changes in HPR width by changing the slope of their rate-level curves. Using realistic single-neuron models, we go on to show that after-hyperpolarizing currents, such as those carried by K(Ca)(2+) channels, may be responsible for the threshold shifts, but not the slope changes.

Vertical eye movement-related responses of neurons in midbrain near intestinal nucleus of Cajal.

Vertical eye movement-related responses of neurons in midbrain near intestinal nucleus of Cajal.