the object that is most relevant to one's behavior. Functional magnetic resonance imaging (fMRI) has shown that biased competition theory can explain the observed attention effects at a neuronal level. Attention effects bias the internal weight (strengthens connections) of task relevant features toward the attended object. This was shown by Reddy, Kanwisher, and van Rullen who found an increase in oxygenated blood to a specific neuron following a locational cue. Further neurological support comes from neurophysiological studies which have shown that attention results from Top-down biasing, which in turn influences neuronal spiking. In sum, external inputs affect the Top-down guidance of attention, which bias specific neurons in the brain. Empirical Evidence for Biased Competition Theory Since the development of Biased Competition Theory much research has been undertaken to validate the theory's ideas. Research has formulated predictions based on the theoretical model to be tested in experimental settings. Top-down and Bottom-up biases have been tested in order to ascertain the legitimacy of their influence on visual perception and competition as described by Biased Competition Theory. These experiments provide evidence for the idea of feedback biasing as described in Desimone's five central tenets. Evidence of Bottom-up Process Bottom-up processes are characterized by an absence of higher level direction in sensory processing. It primarily relies on sensory information and incoming sensory information is the starting point for all Bottom-up processing. Bottom-up refers to when a feature stands out in a visual search. This is commonly called the “pop-out” effect. Salient features like bright colors, movement and big objects make the object “pop-out” of the visual search. “Pop-out” features can often attract attention without conscious processing. Objects that stand out are often given priority (bias) in processing. Bottom-up processing is data driven, and according to this stimuli are perceived on the basis of the data which is being experienced through the senses. Evidence suggests that simultaneously presented stimuli do in fact compete in order to be represented in the visual cortex, with stimuli mutually suppressing each other to gain this representation. This was examined by Reynolds and colleagues, who looked at the size of neurons’ receptive field's within the visual cortex. It was found that the presentation of a single stimulus resulted in a low firing rate while two stimuli presented together resulted in a higher firing rate. Reynolds and colleagues also found that when comparing the neural response of an individually presented visual