Valoración Bourdiana y la propuesta de una sociología reflexiva

Motion sensitivity of cortical units manifested as strong facilitation of responses to dynamic stimulation in contrast to static stimulation has not been described in bats before. Facilitated neuronal responses in bats have been recorded to combinations of time shifted spectral elements of the bat’s echolocation call in the inferior colliculus of P. parnellii (Mittmann and Wenstrup 1995), in the medial geniculate body of P. parnellii (Olsen and Suga 1991b) and in the AC of R. rouxi and P. parnellii (Suga and Horikawa 1986; Schuller et al. 1991) and of

M. lucifugus (Tanaka et al. 1992). Delay-tuned neurons activated by combinations of FM components of the echolocation call and echo encode target range (Feng et al. 1978; O'Neil and Suga 1979), whereas neurons with facilitatory responses to combinations of the CF part of the call and the echo may be involved in identifying the velocity of a sound source moving towards the animal in a radial direction (Suga 1989; Olsen and Suga 1991a). Facilitated responses in these studies were related to the temporal sequence of the different spectral components, and the spatial position of the stimulus source remained constant. In the present study, the spectral content of the stimulus components remains constant since pure tones at BF of the particular unit were used. Furthermore, the spatial positions with the maximum dynamical response of motion sensitive units (black asterisks in Fig. 4.8D and 4.9B, H and L) never coincided with positions on the diagonal line of the dynamic response area designating stationary two-tone stimuli. Thus, facilitation was strongest when two successive stimuli were separated in spatial position as well as in time, and therefore essentially resulted from acoustic motion in space. However, as shown for delay tuned combination sensitive neurons, in the AC of P. parnellii (Suga 1990), R. rouxi (Schuller et al. 1991) and C. perspicillata (Esser and Eiermann 2004), motion sensitive neurons with strongly facilitated responses in P. discolor

were located in the dorsal region of the AC above the tonotopically organized primary auditory areas. But in contrast to delay tuned combination sensitive neurons in the AC of the closely related phyllostomid bat C. perspicillata, motion sensitive neurons with facilitatory responses in Phyllostomus were not equally distributed in dorsal cortical subfields but were confined to the caudal sub-region of the dorsal AC, the posterior dorsal field (PDF). This indicates that the PDF is especially involved in processing of acoustic motion and supports the finding of a previous study investigating the organization of basic response properties in the AC of P. discolor where PDF was supposed to be particularly important for processing of information derived from echolocation (Hoffmann et al. 2008b). Concluding, it is obvious that motion sensitivity in the AC of Phyllostomus is somehow related to combination sensitivity in the AC of other bats. But whereas combination sensitivity in the previous

studies is simply an effect of delay tuning, motion sensitivity in the present study is an effect of delay tuning with a compulsive spatial component.

Up to now, motion sensitivity was always characterized either by a shift of the azimuthal response area during stimulation with moving sounds or by a preference for a certain motion direction (Sovijärvi and Hyvärinen 1974; Wagner and Takahashi 1990; Ahissar et al. 1992; Kleiser and Schuller 1995; Wilson and O'Neill 1998; Firzlaff and Schuller 2001b; Schlegel 2002). However, these criteria did not serve for distinguishing motion sensitive from insensitive units in the present study, because units of both types showed response area shifts and preferences for a certain motion direction. The size and spatial position of the azimuthal response area of units in the present study was always influenced by the IPI. In motion sensitive units the azimuthal response area increased with decreasing IPI and thus was expanded towards previously unresponsive positions. With further decreasing IPI the azimuthal response area decreased again and at shortest IPIs often focused on small distant movements passing frontal azimuthal positions. Thus, these neurons seem to be presumably relevant for the detection of moving targets during the phase of final approach towards a target when the repetition rate of emitted calls and returning echoes, respectively, is highest. This point of view is supported by a study on the sonar beam pattern of E. fuscus (Ghose and Moss 2003). Ghose and Moss described that the bat first scans its environment with the sonar beam. After a target was selected, Eptesicus increases the repetition rate of its calls and centers the axis of its sonar beam at the target while approaching it. The angle between the beam axis and the target progressively decreases with decreasing time to contact, indicating that the bat first perceives the target at peripheral positions of its acoustic gaze and when it gets closer to the target, the bat actively focuses its sonar beam onto the target. At a time to contact of 300 ms, the bat locked its beam onto the target with an accuracy of ±3 °. At this point in time, the IPI between successively emitted calls and returning echoes, respectively, is below 50 ms and it further decreases with decreasing time to contact. Ghose and Moss hypothesize, that the motor action of focusing the sonar beam to the direction of a target is a naturally occurring behavior and is part of the target selection and tracking process. The motion sensitive cortical units in the present study, which focused their azimuthal response area on small movements in the frontal field at short IPIs could probably provide acoustic feedback information for the vocal motor action. For example, the unit depicted in Fig. 4.8F-I responded strongest to largely scaled movements starting at peripheral azimuthal positions (-40 to -30 and 30 to 40 dB IID) at an IPI of 25 ms (scanning phase) and focused its azimuthal response area to small movements in the frontal field at an IPI of 6.25 ms. The hypothesis is further supported by the distribution of best IPIs in motion sensitive units. In these units best IPIs were never above 50 ms, which is the IPI occurring between emitted vocalizations 300 ms before contact with the target, i.e. the point in time when the bat locked its beam axis onto the selected target (Ghose and Moss 2003).