We investigated the responses of single neurons in the middle temporal area (MT) of anesthetized marmoset monkeys to sine wave gratings of various lengths and widths. For the vast majority of MT cells maximal responses were obtained upon presentation of gratings of specific dimensions, which were typically asymmetrical along the length and width axes. The strength of end-inhibition was dependent on the width of the stimulus, with many cells showing clear end-inhibition only when wide gratings were used. Conversely, the strength of side-inhibition was dependent on stimulus length. Furthermore, for over one-third of MT cells length summation properties could not be defined without consideration of stimulus width, and vice-versa. These neurons, which we refer to as length-width inseparable (LWI) cells, were rare in layer 4. The majority of LWI neurons was strongly inhibited by wide-field stimuli and responded preferentially to gratings that were elongated, along either the length or width dimensions. However, rather than forming a homogenous and entirely distinct group, LWI cells represented the upper end of a continuum of complexity in spatial summation response properties, which characterized the population of MT cells. Only a minority of MT neurons (22.3%) showed no evidence of inhibition by wide-field stimuli, with this type of response being common among layer 5 cells. These results demonstrate distinct patterns of spatial selectivity in MT, supporting the notion that neurons in this area can perform various roles in terms of grouping and segmentation of motion signals.