Page 30 - It' about time: Studying the Encoding of Duration
P. 30

                                An investigation of the spatial selectivity of the duration after-effect  example, it has been shown that adapting to the temporal frequency content of a stimulus can cause spatially localized shifts in the perceived duration of subsequent events. These spatially selective after-effects following adaptation have been attributed to modulation in neurons in LGN and V1 (Ayhan, Bruno, Nishida, & Johnston, 2009; Johnston et al., 2006; Ortega et al., 2012; but see Burr, Tozzi, & Morrone, 2007; Fornaciai, Arrighi, & Burr, 2016). These findings suggest a strong relation between low-level visual processing and the encoding of 2 temporal information. The above studies suggest that duration information is processed at early levels of processing. However, behavioral studies have shown that trial history effects and the duration after-effect do not show any selectivity to low-level visual features such as orientation (Li, Yuan, & Huang, 2015; Walker et al., 1981), arguing against a role of early visual cortex in channel-based duration processing. Furthermore, many other different brain areas have been implicated in duration processing, providing alternative possible neural loci for the channel-based encoding of duration (Hayashi et al., 2015; Ivry & Schlerf, 2008; Mauk & Buonomano, 2004). For example, single cell recordings in macaques have revealed duration selectivity in striatal neurons (Mello, Soares, & Paton, 2015) as well as in the pre-supplementary motor area (pre-SMA) (Merchant, Pérez, et al., 2013). More recently, Hayashi et al. (2015)investigated single duration repetitions in humans using fMRI. They showed a decrease in BOLD response in the right-supramarginal gyrus (r-SMG) when the duration of a stimulus was similar to a previously presented stimulus, as compared to when both were dissimilar. This suppressed response to repetition was replicated for several different intervals and did not seem to be the result of a general similarity judgment, only occurring for duration judgments (Hayashi et al., 2015). In sum, it is clear that the mechanisms involved in duration processing and their related structures in the human brain are yet to be established. The goal of this study was to further investigate the relative position of duration-selective mechanisms along the visual processing hierarchy by investigating the spatial selectivity of the duration after-effect. It is well known that the spatial scale over which sensory information is integrated increases along the visual processing hierarchy (A. T. Smith et al., 2001). This is the result of differences in receptive field size of individual neurons in different cortical areas. For early visual areas such as V1, receptive fields have been found to be as small as 0.5°, with the  29 


































































































   28   29   30   31   32