Composición de los gases de y calculo del flujo masico.

The importance of multiple objects in the visual field is a complex and interdisci- plinary issue that incorporates many aspects described in the previous sections. For instance, contour integration can be considered a response to the numerosity of local features, while attention can be considered to be an ecological strategy to function adaptively in the context of many objects.

The next three subsections are concerned with the importance of multiple objects and features to the visual system and will demonstrate that the numerosity of ob- jects in the visual field is not accidental to the functioning of the visual system, but is a key component of visual processes and needs to be investigated to develop a fuller understanding of the underlying behaviour that leads to our experience of the world.

Redundancy gain from multiple visual features

The detection of an otherwise un-obscured object by a visual system should seem like a rather trivial process unlike the detection of illusory objects. However, early work on inter-sensory e↵ects by Todd (1912) showed that the speed by which a detection task is performed by an observer could be modulated by the simultaneous presentation of two otherwise separate ’signals’ (e.g., an auditory tone and a light). More specifically, the mean response time of an observer to a target was lower when both the tone and light were presented in comparison to the presentation of either the tone or light alone.

In the visual modality, a similar e↵ect has been observed between single feature dimensions of a singleton target being searched for by an observer. In such visual search tasks observers are presented with a large number of possible objects, with the specific target di↵ering from the distracters by being orientated; coloured or orientated and coloured. As with the inter-modal e↵ects, additional features related

to the central target detection decreased the mean response time for such targets (Miller, 1982; Toellner, Zehetleitner, Krummenacher, & Mueller, 2011; Krumme- nacher, Muller, & Heller, 2001, 2002a, 2002b; Ivanov & Werner, 2009; Grubert, Krummenacher, & Eimer, 2011).

The redundancy gain provided by the simultaneous presentation of other relevant features has been shown to be sensitive to the presence of higher-level semantic associations. For instance, an individual letter in the modern Latin alphabet has an upper case and lower case letter for a single phoneme. Ben-David and Algom (2009) presented a target letter (say, the letter ’a’) with adjacent flanking letters that had identical visual features (’a’); were related by sharing a semantic role (’A’) or varying in shape and semantic meaning (’b’ and ’B’). By measuring the mean reaction time corresponding to successful and accurate detection they determined that the presence of both identical visual features and higher-level semantic meaning decreased the mean reaction time.

By using measurements of how quickly target detection occurs, the redundant sig- nals e↵ect is used to investigate how additional information from the various sensory modalities is combined in the act of detecting a target. At a very basic level then, any experiment that presents multiple simultaneous cues may be invoking the tem- poral benefits in combining two or more signals together.

The present thesis presents multiple ’signals’ (objects or contours) to determine whether they play a role in perceptual organisation. Hence, the focus of the thesis di↵ers from redundancy investigations in that it attempts to determine the benefits or detriments to spatial processes (e.g., integration of a contour across the visual field) rather than the temporal benefits of numerosity to detection (e.g., The latency of detection processes). However, it is clear that these two aspects - the temporal and spatial impact of multiplicity - are likely complementary and worthy of future joint investigation.

Encoding of sets of objects

The quantification of objects is often associated with a single value such as the specific orientation of an object with respect to some axis. However, as is used so frequently in science, valuable information can be encoded by taking the average value of group of objects. Humans are remarkably adept at making guesses and judgements about general features in the world (e.g., clouds are white) but less adept at deciphering the specific features (e.g., the complex and sometimes subtle patterns of graduation of luminance and colour in clouds.)

Alvarez (2011) investigated the capacity of observers to make judgements about the general and specific features of sets of objects. Judgements concerning the gen- eral features of sets of objects were significantly accurate when compared with the true mean of the set. However, unlike the largely accurate judgements of the mean of some feature, observers were less capable of accurately accessing the individual features involved. This process of encoding the mean values of a feature of a set of objects, or ensemble encoding, was demonstrated to occur for a variety of features such as size (Ariely, 2001; Chong & Treisman, 2003), orientation (Dakin & Watt, 1997; Chong & Treisman, 2003; Parkes, Lund, Angelucci, Solomon, & Morgan, 2001) and the position (Alvarez & Oliva, 2008).

Though primarily focused on cognitive judgements about visual features these be- havioural experiments demonstrate that general information derived from sets of objects are available in the visual system. Hence, the human visual system is in some sense functionally designed to process the general information from a scene that may pertain to a group of objects, but may not be necessarily tuned to highly specific information.

Facilitation, Surround suppression and crowding in local features

As has been described in the previous sections, often the neurons in some region of the visual cortex are tuned to specific features and have specific receptive field sizes (see Sections 1.2.1, p.6). However, as with any general process that responds

across a specific region, environmental circumstances may lead to congruent and conflicting features lying in a single visual field.

As described in Section 1.2.1, enhancements to the detectability of a localised tar- get feature have been observed when presented in the presence of additional flankers (Polat & Sagi, 1993; Adini et al., 1997; Zenger & Sagi, 1996; Bonneh & Sagi, 1999; Churan et al., 2009; Cass & Spehar, 2005; Chen & Tyler, 2001; Freeman et al., 2001; Huang & Hess, 2007; Mizobe et al., 2001; Katkov & Sagi, 2010; Polat & Tyler, 1999; Sterkin et al., 2008; Woods et al., 2002). Likewise, a single increased size of local feature, or multiple features in close proximity have been shown to have the oppo- site, suppressive e↵ect in which the central target region becomes less detectable when the adjacent regions are simultaneously occupied by some competing flank- ing stimuli. This e↵ect is known as psychophysical surround suppression (Tadin, Lappin, Gilroy, & Blake, 2003; Born, 2000; Pack, Hunter, & Born, 2005; Churan et al., 2009; Spillmann, 1994; Troncoso et al., 2007; Petrov, Popple, & McKee, 2007). Other e↵ects can inhibit the recognition of features, in what is described as crowd- ing e↵ects (Bouma, 1970; Stuart & Burian, 1962; Pelli & Tillman, 2008; Toet & Levi, 1992; Levi, 2008; Levi, Hariharan, & Klein, 2002; Parkes et al., 2001; Pelli, Palomares, & Majaj, 2004). Under such conditions the capacity to distinguish the orientation or configuration of a target is a↵ected by the presentation of conflicting flanker information.

The underlying activity of neurons in the visual cortex under such conditions has been well documented in a large number of studies (Kastner, De Weerd, Desimone, & Ungerleider, 1998; Kastner et al., 2001; Desimone & Duncan, 1995; D. M. Beck & Kastner, 2009; Joo, Boynton, & Murray, 2012). Desimone and Duncan (1995) posited that the suppressive e↵ect on activity is due to features competing for the neurons response. However, most recently, the process of presenting redundant in- formation in the presence of a central contrast detection task actively facilitates and increases the activity of certain regions of the visual cortex (Shim, Jiang, & Kanwisher, 2013). This ’redundancy signal gain’ may indicate that the most recent

understanding of the role of suppression and inhibition of multiple objects and fea- tures activating individual or groups of neurons in the visual system may be more complex than previous research has indicated.

Summary of neurophysiological of early visual system

As has been described elsewhere in the introduction (p.3) , one significant task that the visual cortex needs to perform is to integrate the local luminance changes across the retina into a coherent whole object (Wertheimer, 1923; Wallach, 1935; van Rossum & Smith, 1998; Laughlin, 1994, 1996). However, even at this stage of processing the response to luminance on the retina is spatially dependent (Regan & Beverley, 1983), for example, the sensitivity to luminance contrast can vary with eccentricity from the foveal region of the eye.

This disparate information is then passed onto the primary visual system (V1 area). Here, neurons are tuned to specific features (e.g., contrast, frequency, orientation) within a given topographic region of the retina (Wertheimer, 1923; Gilbert & Wiesel, 1979, 1983; Marcelja, 1980). When the spatial distribution of such local features is taken into account, more complex visual features - such as curvature - are formed, and these are represented in regions beyond V1. Changes in curvature, for exam- ple, have been shown to trigger neuronal responses in the v2 region of the cortex (Blakemore & Over, 1974; Watt & Andrews, 1982; Ho↵man & Richards, 1984). Neurons further in the processing hierarchy are shown to be sensitive to even more global properties such as shape circularity. Sensitivity to circularity is associated with the v4 area of visual cortex (Gallant, Braun, & Vanessen, 1993; Gallant, Con- nor, Rakshit, Lewis, & VanEssen, 1996; Wilkinson et al., 2000; Wilson & Wilkinson, 1998; Dumoulin & Hess, 2007).