Auditory selective attention (ASA) is a cognitive process that identifies and focuses attention on relevant sounds in a noisy environment. The auditory steady-state response (ASSR) is a technique used to assess ASA using unimodal sound stimuli. The present study investigates ASA with ASSR during unimodal auditory stimulation. Carrier tones of 500 Hz (modulated at 31 Hz) and 2000 Hz (modulated at 39 Hz) were applied to the left (LE) and right (RE) ears, respectively. EEG signals from 18 participants with normal hearing were recorded during binaural stimulation with a sound pressure of 75 dBSPL, in three conditions: No Attention; RE Attention and; LE Attention. The Magnitude Squared of Coherence (MSC) was used to identify ASSR. An attention index with MSC was proposed to evaluate the different attention conditions. The results of the unimodal auditory SA reinforce previous findings of laterality in the right hemisphere with an increase in the ipsilateral response (p = 0.0301) to the RE sound stimulus (39 Hz) compared to attention to the LE (31 Hz).