LA PARTICIPACIÓN JUVENIL

Classical Encoders are the predominant response type, and are mostly X-like cells (Fig. 3.12). X cells are characterized by sustained response and also responded with a preferred and null phase to reversing gratings (Enroth-Cugell and Robson 1966, Stein et al. 1983). Thus, the response profile of Classical Encoders, which are also spatial phase dependent and have sustained firing (Fig. 3.2 A, B), is easily explained by their basic physiological property.

Response types other than Classical Encoders are mostly Y-like cells, which respond with ‘bursty’ firing to fast moving gratings (Stein et al. 1983). This property explains the transient firing during saccade-like scene shifts. But Y cell properties alone could not explain the response to fixated images. ForChange Detectors, long-lasting adapta- tion might cause the response attenuation if the same stimulus appears after a saccade. Synaptic facilitation may cause enhanced response inSimilarity Detectors if the same

74 6.3. Comparison with Other Classification

stimulus appears after a saccade. Alternatively, there may be a common circuitry that differentially modulates these two types simultaneously, producing opposite response patterns. More than half of Indifferent Encoders are ON-OFF cells, thus explaining their response to all of the fixated images. Offset Detectors may have a similar mech- anism to local edge detectors (LED) which respond only to moving edges and not to fixated images.

6.3

Comparison with Other Classification

Retinal ganglion cells of mouse have been classified extensively by several research groups based on morphology, physiology, dendritic arborisation, dendritic thickness, molecular and genetic markers, projection to different brain regions etc (Doi et al. 1995, Sun et al. 2002, Carcieri et al. 2003, Badea and Nathans 2004, Kong et al. 2005, V¨olgyi et al. 2005, 2009, Coombs et al. 2006, Hattar et al. 2006, Loopuijt et al. 2007, Huberman et al. 2008, 2009, Kim et al. 2008, Siegert et al. 2009, Yonehara et al. 2008, 2009, 2011, Farrow and Masland 2011, Hong et al. 2011, Kay et al. 2011, Rivlin-Etzion et al. 2011). The five response types described in our study arise when ganglion cells were tested with a specific type of stimulus namely the saccade stimulus. We sought to look into some of the earlier classifications and see how well our classification fits or deviates from the existing schema.

Comparison with anatomical/morphological classification: There are at least 12 types of ganglion cells based on anatomical classification alone (Badea and Nathans 2004, Kong et al. 2005, Coombs et al. 2006, V¨olgyi et al. 2009).We wanted to know if each response type belongs to a specific anatomical or morphological cell class. In our experiments we do not have access to the morphology of a ganglion cell. Nevertheless, the receptive field size of a ganglion cell estimated by reverse correlation (see Section 2.6.3; Fig. 2.4) offers some clue about a cell’s dendritic field size. Our results show that each response type comprises cells with different receptive field sizes (Fig. 3.11). Classical Encoders

were mostly small cells (≈150µm) though some large cells were also found. The other response types are even more heterogeneous, comprising cells with different receptive field sizes . Thus it is clear that a response type may contain more than one anatomical cell class.

Comparison with functional classification: We used a nonlinearity index to classify cells into X-like and Y-like. Although we found X-like and Y-like cells, they did not form separate group of cells, rather formed a continuum as reported by Carcieri et al. (2003).

of the cells, on the other hand, were mostly Y-like, although some X-like cells were present. Y-like cells in mouse are known to have three anatomical subtypes based on their electrical coupling patterns (V¨olgyi et al. 2005). It is not clear if each of these subtypes corresponds to different response types described in our study.

Furthermore we also identified direction selective and orientation selective cells in our recordings, but they do not cluster into a single response type.

While it is difficult to assign different cell classes to different response types, one cell class stands out - X-like cells which have small receptive field (≈150µm), brisk sustained response pattern, also known as beta cell (based on soma size). This cell type form ≈70% ofClassical Encoders. Y-like cells are characterized by medium to large receptive fields and brisk transient response. They are known as alpha cells (based on soma size) and form majority of rest of response types Offset Detector, Indifferent Encoder

and Similarity Detector. The cells of Change Detector have slightly different response pattern indicating a non-X and non-Y cell type. The response profile of the cells (8 out of 9 cells) was neither transient nor sustained (Fig.3.5) with a lower peak firing rate. Thus we could identify at least three physiological cell classes with each forming predominantly but not exclusively three groups of response types. These three broad physiological cell classes may correspond to S-units (mostly X cells), T-units (mostly Y cells) and M-units described in cat retina during a saccade, described by (Noda 1975). While there are no OFF cells in Change Detector, there are no ON cells in Similarity Detector. These are two opposite response types and most likely opposite cell types. In guinea pig retina, it is common to find an ON cell - OFF cell pair (unpublished observation). It is likely that they may exist in mouse retina as well and function as

Change Detector - Similarity Detector cell pair.

6.4

Does the Retina Contribute to Saccadic Suppres-

sion?

During saccadic eye movements, the image on the retina is highly blurred, but our visual perception to this motion blur is suppressed. This process is known as ‘saccadic suppression’ and it is an important phenomenon, so that we can perceive a stable vi- sual world (Volkmann 1962, Beeler 1967, Noda and Adey 1974a, Burr et al. 1994, Ross et al. 2001). Saccadic suppression has been observed in several species including rodents (Lee et al. 2007, Phongphanphanee et al. 2011), cat (Noda and Adey 1974a), monkey (Ross et al. 2001) and human (Beeler 1967). It is also observed at different stages in