Neural Mechanisms Underlying the Perception of Three-Dimensional Shape from Texture: Adaptation and Aftereffects

Input into the visual system is two-dimensional (2D) and yet we effortlessly perceive the world around us as three-dimensional (3D). How we are able to accurately extract 3D shape information from the 2D representations that fall on the retina remains largely unknown. Although much research has been conducted that investigates higher levels of form processing (i.e. face recognition), less is known about the mechanisms that underlie the perception of simple 3D shape. Previous studies in our lab have shown that our ability to perceive 3D shape from texture cues relies on the visibility of orientation flows -- patterns that run parallel to the surface curvature of a 3D shape. Using the psychophysical technique of selective adaptation, we have further characterized the neural mechanisms that underlie the accurate perception of 3D shape. In Experiment One, we examined whether orientation flows that are defined by second order contours convey 3D shape, whether they induce 3D shape aftereffects, and whether these aftereffects are invariant to the patterns that define the orientation flows. Aftereffects were obtained and 3D shape was conveyed using stimuli in which orientation flows were defined by two classes of second order contours, and adapting to second order stimuli caused 3D shape aftereffects in first order stimuli. These results can be explained by the adaptation of 3D shape-selective neurons in extrastriate regions that invariantly extract first- and second order orientation flows from striate and extrastriate signals. In Experiment Two, we were interested in determining to what extent these neural mechanisms are invariant to differences in spatial frequency. We chose adapting/test stimuli that differed in spatial frequency by a factor of three, consistent with documented frequency bandwidths of V1 and V2 neurons. Shape aftereffects were obtained, indicating that these neural mechanisms are invariant to differences in spatial frequency by a factor of 3. Furthermore, these neural mechanisms are invariant to the patterns in which spatial frequency was varied (i.e., stimuli in which the orientation flows were created by first- or second order properties). Both of these properties are indicative of neurons that are located in extrastriate cortex. In Experiment Three, we were interested in testing to what extent these neural mechanisms were selective for retinal position by misaligning adapting and test stimuli by 2°, which corresponded to a single convexity or concavity in our corrugated surfaces. Our results suggest that 3D shape-selective mechanisms that respond to luminance modulated orientation flows appear to be sensitive to shifts in position of 2°. Overall, our results indicate that there are 3D shape mechanisms that are pattern invariant, invariant to differences in spatial frequencies by a factor of 3, and that exhibit position selectivity to shifts in retinal position of 2°. Taken together, these results implicate 3D shape mechanisms that are located in extrastriate cortex

La canonica di San Lorenzo a Genova. Dinamiche istituzionali e rapporti sociali (secoli X-XII)

Studiare la comunità canonicale genovese, dalle prime testimonianze relative alla presenza di un nucleo di convivenza sacerdotale presso la chiesa vescovile (metà del secolo X) alla definitiva affermazione – in un contesto ormai comunale e arcidiocesano – dell’istituzione identificata come Capitulum: sono questi gli obiettivi perseguiti dal saggio, che propone un percorso attraverso la definizione dei rapporti con il vescovo, della gerarchia interna alla comunità, del patrimonio fondiario e immobiliare, della propria identità rispetto al comune e ai cives, per tentare di illustrare le dinamiche che caratterizzano i primi tre secoli di vita istituzionale della canonica della chiesa vescovile di Genova

La Science de la Legislation ...

Tomos 4º y 5

Ciencia de la legislación

Tomos IV, 1ª y 2ª parte y tomo

La science de la législation ... ; tome quatrieme

Sign.: A-R\p8\sPort. con grav. xi

La Science de la Legislation

Tomos 1º y 2